Today, friends, there will be no snot or sentimentality. Instead, I will send you, no questions asked, into battle with one of the most formidable opponents in the 8th-9th grade algebra course.
Yes, you understood everything correctly: we are talking about inequalities with modulus. We will look at four basic techniques with which you will learn to solve about 90% of such problems. What about the remaining 10%? Well, we'll talk about them in a separate lesson. :)
However, before analyzing any of the techniques, I would like to remind you of two facts that you already need to know. Otherwise, you risk not understanding the material of today’s lesson at all.
Captain Obviousness seems to hint that to solve inequalities with modulus you need to know two things:
Let's start with the second point.
Everything is simple here. There are two definitions: algebraic and graphical. To begin with - algebraic:
Definition. The modulus of a number $x$ is either the number itself, if it is non-negative, or the number opposite to it, if the original $x$ is still negative.
It is written like this:
\[\left| x \right|=\left\( \begin(align) & x,\ x\ge 0, \\ & -x,\ x \lt 0. \\\end(align) \right.\]
Speaking in simple language, the modulus is “a number without a minus”. And it is in this duality (in some places you don’t have to do anything with the original number, but in others you have to remove some kind of minus) that is where the whole difficulty lies for beginning students.
There is also a geometric definition. It is also useful to know, but we will turn to it only in complex and some special cases, where the geometric approach is more convenient than the algebraic one (spoiler: not today).
Definition. Let point $a$ be marked on the number line. Then the module $\left| x-a \right|$ is the distance from point $x$ to point $a$ on this line.
If you draw a picture, you will get something like this:
Graphical module definition One way or another, from the definition of a module its key property immediately follows: the modulus of a number is always a non-negative quantity. This fact will be a red thread running through our entire narrative today.
Now let's look at the inequalities. There are a great many of them, but our task now is to be able to solve at least the simplest of them. Those that reduce to linear inequalities, as well as to the interval method.
I have two big lessons on this topic (by the way, very, VERY useful - I recommend studying them):
If you know all this, if the phrase “let’s move from inequality to equation” does not make you have a vague desire to hit yourself against the wall, then you are ready: welcome to hell to the main topic of the lesson. :)
This is one of the most common problems with modules. It is required to solve an inequality of the form:
\[\left| f\right| \ltg\]
The functions $f$ and $g$ can be anything, but usually they are polynomials. Examples of such inequalities:
\[\begin(align) & \left| 2x+3 \right| \lt x+7; \\ & \left| ((x)^(2))+2x-3 \right|+3\left(x+1 \right) \lt 0; \\ & \left| ((x)^(2))-2\left| x \right|-3 \right| \lt 2. \\\end(align)\]
All of them can be solved literally in one line according to the following scheme:
\[\left| f\right| \lt g\Rightarrow -g \lt f \lt g\quad \left(\Rightarrow \left\( \begin(align) & f \lt g, \\ & f \gt -g \\\end(align) \right.\right)\]
It is easy to see that we get rid of the module, but in return we get a double inequality (or, which is the same thing, a system of two inequalities). But this transition takes into account absolutely everything possible problems: if the number under the modulus is positive, the method works; if negative, it still works; and even with the most inadequate function in place of $f$ or $g$, the method will still work.
Naturally, the question arises: couldn’t it be simpler? Unfortunately, it's not possible. This is the whole point of the module.
However, enough with the philosophizing. Let's solve a couple of problems:
Task. Solve the inequality:
\[\left| 2x+3 \right| \lt x+7\]
Solution. So, we have before us a classic inequality of the form “the modulus is less” - there’s even nothing to transform. We work according to the algorithm:
\[\begin(align) & \left| f\right| \lt g\Rightarrow -g \lt f \lt g; \\ & \left| 2x+3 \right| \lt x+7\Rightarrow -\left(x+7 \right) \lt 2x+3 \lt x+7 \\\end(align)\]
Do not rush to open the parentheses preceded by a “minus”: it is quite possible that due to your haste you will make an offensive mistake.
\[-x-7 \lt 2x+3 \lt x+7\]
\[\left\( \begin(align) & -x-7 \lt 2x+3 \\ & 2x+3 \lt x+7 \\ \end(align) \right.\]
\[\left\( \begin(align) & -3x \lt 10 \\ & x \lt 4 \\ \end(align) \right.\]
\[\left\( \begin(align) & x \gt -\frac(10)(3) \\ & x \lt 4 \\ \end(align) \right.\]
The problem was reduced to two elementary inequalities. Let us note their solutions on parallel number lines:
Intersection of many
The intersection of these sets will be the answer.
Answer: $x\in \left(-\frac(10)(3);4 \right)$
Task. Solve the inequality:
\[\left| ((x)^(2))+2x-3 \right|+3\left(x+1 \right) \lt 0\]
Solution. This task is a little more difficult. First, let’s isolate the module by moving the second term to the right:
\[\left| ((x)^(2))+2x-3 \right| \lt -3\left(x+1 \right)\]
Obviously, we again have an inequality of the form “the module is smaller”, so we get rid of the module using the already known algorithm:
\[-\left(-3\left(x+1 \right) \right) \lt ((x)^(2))+2x-3 \lt -3\left(x+1 \right)\]
Now attention: someone will say that I'm a bit of a pervert with all these parentheses. But let me remind you once again that our key goal is correctly solve the inequality and get the answer. Later, when you have perfectly mastered everything described in this lesson, you can pervert it yourself as you wish: open parentheses, add minuses, etc.
To begin with, we’ll simply get rid of the double minus on the left:
\[-\left(-3\left(x+1 \right) \right)=\left(-1 \right)\cdot \left(-3 \right)\cdot \left(x+1 \right) =3\left(x+1 \right)\]
Now let's open all the brackets in the double inequality:
Let's move on to the double inequality. This time the calculations will be more serious:
\[\left\( \begin(align) & ((x)^(2))+2x-3 \lt -3x-3 \\ & 3x+3 \lt ((x)^(2))+2x -3 \\ \end(align) \right.\]
\[\left\( \begin(align) & ((x)^(2))+5x \lt 0 \\ & ((x)^(2))-x-6 \gt 0 \\ \end( align)\right.\]
Both inequalities are quadratic and can be solved by the interval method (that’s why I say: if you don’t know what this is, it’s better not to take on modules yet). Let's move on to the equation in the first inequality:
\[\begin(align) & ((x)^(2))+5x=0; \\ & x\left(x+5 \right)=0; \\ & ((x)_(1))=0;((x)_(2))=-5. \\\end(align)\]
As you can see, the output is an incomplete quadratic equation, which can be solved in an elementary way. Now let's look at the second inequality of the system. There you will have to apply Vieta’s theorem:
\[\begin(align) & ((x)^(2))-x-6=0; \\ & \left(x-3 \right)\left(x+2 \right)=0; \\& ((x)_(1))=3;((x)_(2))=-2. \\\end(align)\]
We mark the resulting numbers on two parallel lines (separate for the first inequality and separate for the second):
Again, since we are solving a system of inequalities, we are interested in the intersection of the shaded sets: $x\in \left(-5;-2 \right)$. This is the answer.
Answer: $x\in \left(-5;-2 \right)$
I think that after these examples the solution scheme is extremely clear:
A similar algorithm exists for inequalities of the following type, when the modulus is greater than the function. However, there are a couple of serious “buts”. We’ll talk about these “buts” now.
They look like this:
\[\left| f\right| \gtg\]
Similar to the previous one? It seems. And yet such problems are solved in a completely different way. Formally, the scheme is as follows:
\[\left| f\right| \gt g\Rightarrow \left[ \begin(align) & f \gt g, \\ & f \lt -g \\\end(align) \right.\]
In other words, we consider two cases:
In this case, the options are combined with a square bracket, i.e. We have before us a combination of two requirements.
Please note again: this is not a system, but a totality, therefore in the answer the sets are combined rather than intersecting. This is a fundamental difference from the previous point!
In general, many students are completely confused with unions and intersections, so let’s sort this issue out once and for all:
To make it even easier to remember, just draw legs to these signs to make glasses (just don’t now accuse me of promoting drug addiction and alcoholism: if you are seriously studying this lesson, then you are already a drug addict):
Difference between intersection and union of sets Translated into Russian, this means the following: the union (totality) includes elements from both sets, therefore it is in no way less than each of them; but the intersection (system) includes only those elements that are simultaneously in both the first set and the second. Therefore, the intersection of sets is never larger than the source sets.
So it became clearer? That is great. Let's move on to practice.
Task. Solve the inequality:
\[\left| 3x+1 \right| \gt 5-4x\]
Solution. We proceed according to the scheme:
\[\left| 3x+1 \right| \gt 5-4x\Rightarrow \left[ \begin(align) & 3x+1 \gt 5-4x \\ & 3x+1 \lt -\left(5-4x \right) \\\end(align) \ right.\]
We solve each inequality in the population:
\[\left[ \begin(align) & 3x+4x \gt 5-1 \\ & 3x-4x \lt -5-1 \\ \end(align) \right.\]
\[\left[ \begin(align) & 7x \gt 4 \\ & -x \lt -6 \\ \end(align) \right.\]
\[\left[ \begin(align) & x \gt 4/7\ \\ & x \gt 6 \\ \end(align) \right.\]
We mark each resulting set on the number line, and then combine them:
Union of sets
It is quite obvious that the answer will be $x\in \left(\frac(4)(7);+\infty \right)$
Answer: $x\in \left(\frac(4)(7);+\infty \right)$
Task. Solve the inequality:
\[\left| ((x)^(2))+2x-3 \right| \gt x\]
Solution. Well? Nothing - everything is the same. We move from an inequality with a modulus to a set of two inequalities:
\[\left| ((x)^(2))+2x-3 \right| \gt x\Rightarrow \left[ \begin(align) & ((x)^(2))+2x-3 \gt x \\ & ((x)^(2))+2x-3 \lt -x \\\end(align) \right.\]
We solve every inequality. Unfortunately, the roots there will not be very good:
\[\begin(align) & ((x)^(2))+2x-3 \gt x; \\ & ((x)^(2))+x-3 \gt 0; \\&D=1+12=13; \\ & x=\frac(-1\pm \sqrt(13))(2). \\\end(align)\]
The second inequality is also a bit wild:
\[\begin(align) & ((x)^(2))+2x-3 \lt -x; \\ & ((x)^(2))+3x-3 \lt 0; \\&D=9+12=21; \\ & x=\frac(-3\pm \sqrt(21))(2). \\\end(align)\]
Now you need to mark these numbers on two axes - one axis for each inequality. However, you need to mark the points in the correct order: than larger number, the further we shift the point to the right.
And here a setup awaits us. If everything is clear with the numbers $\frac(-3-\sqrt(21))(2) \lt \frac(-1-\sqrt(13))(2)$ (the terms in the numerator of the first fraction are less than the terms in the numerator of the second , so the sum is also less), with the numbers $\frac(-3-\sqrt(13))(2) \lt \frac(-1+\sqrt(21))(2)$ there will also be no difficulties (positive number obviously more negative), then with the last couple everything is not so clear. Which is greater: $\frac(-3+\sqrt(21))(2)$ or $\frac(-1+\sqrt(13))(2)$? The placement of points on the number lines and, in fact, the answer will depend on the answer to this question.
So let's compare:
\[\begin(matrix) \frac(-1+\sqrt(13))(2)\vee \frac(-3+\sqrt(21))(2) \\ -1+\sqrt(13)\ vee -3+\sqrt(21) \\ 2+\sqrt(13)\vee \sqrt(21) \\\end(matrix)\]
We isolated the root, got non-negative numbers on both sides of the inequality, so we have the right to square both sides:
\[\begin(matrix) ((\left(2+\sqrt(13) \right))^(2))\vee ((\left(\sqrt(21) \right))^(2)) \ \ 4+4\sqrt(13)+13\vee 21 \\ 4\sqrt(13)\vee 3 \\\end(matrix)\]
I think it’s a no brainer that $4\sqrt(13) \gt 3$, so $\frac(-1+\sqrt(13))(2) \gt \frac(-3+\sqrt(21)) (2)$, the final points on the axes will be placed like this:
A case of ugly roots
Let me remind you that we are solving a set, so the answer will be a union, not an intersection of shaded sets.
Answer: $x\in \left(-\infty ;\frac(-3+\sqrt(21))(2) \right)\bigcup \left(\frac(-1+\sqrt(13))(2 );+\infty \right)$
As you can see, our scheme works great for both simple and very tough problems. The only “weak point” in this approach is that you need to correctly compare irrational numbers (and believe me: these are not only roots). But a separate (and very serious) lesson will be devoted to comparison issues. And we move on.
Now we get to the most interesting part. These are inequalities of the form:
\[\left| f\right| \gt\left| g\right|\]
Generally speaking, the algorithm that we will talk about now is correct only for the module. It works in all inequalities where there are guaranteed non-negative expressions on the left and right:
What to do with these tasks? Just remember:
In inequalities with non-negative “tails”, both sides can be raised to any natural power. There will be no additional restrictions.
First of all, we will be interested in squaring - it burns modules and roots:
\[\begin(align) & ((\left(\left| f \right| \right))^(2))=((f)^(2)); \\ & ((\left(\sqrt(f) \right))^(2))=f. \\\end(align)\]
Just don’t confuse this with taking the root of a square:
\[\sqrt(((f)^(2)))=\left| f \right|\ne f\]
Countless mistakes were made when a student forgot to install a module! But this is a completely different story (these are, as it were, irrational equations), so we will not go into this now. Let's solve a couple of problems better:
Task. Solve the inequality:
\[\left| x+2 \right|\ge \left| 1-2x \right|\]
Solution. Let's immediately notice two things:
- This is not a strict inequality. Points on the number line will be punctured.
- Both sides of the inequality are obviously non-negative (this is a property of the module: $\left| f\left(x \right) \right|\ge 0$).
Therefore, we can square both sides of the inequality to get rid of the modulus and solve the problem using the usual interval method:
\[\begin(align) & ((\left(\left| x+2 \right| \right))^(2))\ge ((\left(\left| 1-2x \right| \right) )^(2)); \\ & ((\left(x+2 \right))^(2))\ge ((\left(2x-1 \right))^(2)). \\\end(align)\]
At the last step, I cheated a little: I changed the sequence of terms, taking advantage of the evenness of the module (in fact, I multiplied the expression $1-2x$ by −1).
\[\begin(align) & ((\left(2x-1 \right))^(2))-((\left(x+2 \right))^(2))\le 0; \\ & \left(\left(2x-1 \right)-\left(x+2 \right) \right)\cdot \left(\left(2x-1 \right)+\left(x+2 \ right)\right)\le 0; \\ & \left(2x-1-x-2 \right)\cdot \left(2x-1+x+2 \right)\le 0; \\ & \left(x-3 \right)\cdot \left(3x+1 \right)\le 0. \\\end(align)\]
We solve using the interval method. Let's move from inequality to equation:
\[\begin(align) & \left(x-3 \right)\left(3x+1 \right)=0; \\ & ((x)_(1))=3;((x)_(2))=-\frac(1)(3). \\\end(align)\]
We mark the found roots on the number line. Once again: all points are shaded because the original inequality is not strict!
Getting rid of the modulus sign
Let me remind you for those who are especially stubborn: we take the signs from the last inequality, which was written down before moving on to the equation. And we paint over the areas required in the same inequality. In our case it is $\left(x-3 \right)\left(3x+1 \right)\le 0$.
OK it's all over Now. The problem is solved.
Answer: $x\in \left[ -\frac(1)(3);3 \right]$.
Task. Solve the inequality:
\[\left| ((x)^(2))+x+1 \right|\le \left| ((x)^(2))+3x+4 \right|\]
Solution. We do everything the same. I won't comment - just look at the sequence of actions.
Square it:
\[\begin(align) & ((\left(\left| ((x)^(2))+x+1 \right| \right))^(2))\le ((\left(\left | ((x)^(2))+3x+4 \right| \right))^(2)); \\ & ((\left(((x)^(2))+x+1 \right))^(2))\le ((\left(((x)^(2))+3x+4 \right))^(2)); \\ & ((\left(((x)^(2))+x+1 \right))^(2))-((\left(((x)^(2))+3x+4 \ right))^(2))\le 0; \\ & \left(((x)^(2))+x+1-((x)^(2))-3x-4 \right)\times \\ & \times \left(((x) ^(2))+x+1+((x)^(2))+3x+4 \right)\le 0; \\ & \left(-2x-3 \right)\left(2((x)^(2))+4x+5 \right)\le 0. \\\end(align)\]
Interval method:
\[\begin(align) & \left(-2x-3 \right)\left(2((x)^(2))+4x+5 \right)=0 \\ & -2x-3=0\ Rightarrow x=-1.5; \\ & 2((x)^(2))+4x+5=0\Rightarrow D=16-40 \lt 0\Rightarrow \varnothing . \\\end(align)\]
There is only one root on the number line:
The answer is a whole interval
Answer: $x\in \left[ -1.5;+\infty \right)$.
A small note about the last task. As one of my students accurately noted, both submodular expressions in this inequality are obviously positive, so the modulus sign can be omitted without harm to health.
But this is a completely different level of thinking and a different approach - it can conditionally be called the method of consequences. About it - in a separate lesson. Now let’s move on to the final part of today’s lesson and look at a universal algorithm that always works. Even when all previous approaches were powerless. :)
What if all these techniques don't help? If the inequality cannot be reduced to non-negative tails, if it is impossible to isolate the module, if in general there is pain, sadness, melancholy?
Then the “heavy artillery” of all mathematics comes onto the scene—the brute force method. In relation to inequalities with modulus it looks like this:
So how? Weak? Easily! Only for a long time. Let's see in practice:
Task. Solve the inequality:
\[\left| x+2 \right| \lt \left| x-1 \right|+x-\frac(3)(2)\]
Solution. This crap doesn't boil down to inequalities like $\left| f\right| \lt g$, $\left| f\right| \gt g$ or $\left| f\right| \lt \left| g \right|$, so we act ahead.
We write out submodular expressions, equate them to zero and find the roots:
\[\begin(align) & x+2=0\Rightarrow x=-2; \\ & x-1=0\Rightarrow x=1. \\\end(align)\]
In total, we have two roots that divide the number line into three sections, within which each module is revealed uniquely:
Partitioning the number line by zeros of submodular functions
Let's look at each section separately.
1. Let $x \lt -2$. Then both submodular expressions are negative, and the original inequality will be rewritten as follows:
\[\begin(align) & -\left(x+2 \right) \lt -\left(x-1 \right)+x-1.5 \\ & -x-2 \lt -x+1+ x-1.5 \\ & x \gt 1.5 \\\end(align)\]
We got a fairly simple limitation. Let's intersect it with the initial assumption that $x \lt -2$:
\[\left\( \begin(align) & x \lt -2 \\ & x \gt 1.5 \\\end(align) \right.\Rightarrow x\in \varnothing \]
Obviously, the variable $x$ cannot simultaneously be less than −2 and greater than 1.5. There are no solutions in this area.
1.1. Let us separately consider the borderline case: $x=-2$. Let's just substitute this number into the original inequality and check: is it true?
\[\begin(align) & ((\left. \left| x+2 \right| \lt \left| x-1 \right|+x-1.5 \right|)_(x=-2) ) \\ & 0 \lt \left| -3\right|-2-1.5; \\ & 0 \lt 3-3.5; \\ & 0 \lt -0.5\Rightarrow \varnothing . \\\end(align)\]
It is obvious that the chain of calculations has led us to an incorrect inequality. Therefore, the original inequality is also false, and $x=-2$ is not included in the answer.
2. Let now $-2 \lt x \lt 1$. The left module will already open with a “plus”, but the right one will still open with a “minus”. We have:
\[\begin(align) & x+2 \lt -\left(x-1 \right)+x-1.5 \\ & x+2 \lt -x+1+x-1.5 \\& x \lt -2.5 \\\end(align)\]
Again we intersect with the original requirement:
\[\left\( \begin(align) & x \lt -2.5 \\ & -2 \lt x \lt 1 \\\end(align) \right.\Rightarrow x\in \varnothing \]
And again, the set of solutions is empty, since there are no numbers that are both less than −2.5 and greater than −2.
2.1. And again a special case: $x=1$. We substitute into the original inequality:
\[\begin(align) & ((\left. \left| x+2 \right| \lt \left| x-1 \right|+x-1.5 \right|)_(x=1)) \\ & \left| 3\right| \lt \left| 0\right|+1-1.5; \\ & 3 \lt -0.5; \\ & 3 \lt -0.5\Rightarrow \varnothing . \\\end(align)\]
Similar to the previous “special case”, the number $x=1$ is clearly not included in the answer.
3. The last piece of the line: $x \gt 1$. Here all modules are opened with a plus sign:
\[\begin(align) & x+2 \lt x-1+x-1.5 \\ & x+2 \lt x-1+x-1.5 \\ & x \gt 4.5 \\ \end(align)\]
And again we intersect the found set with the original constraint:
\[\left\( \begin(align) & x \gt 4.5 \\ & x \gt 1 \\\end(align) \right.\Rightarrow x\in \left(4.5;+\infty \right)\]
Finally! We have found an interval that will be the answer.
Answer: $x\in \left(4,5;+\infty \right)$
Finally, one remark that may save you from stupid mistakes when solving real problems:
Solutions to inequalities with moduli usually represent continuous sets on the number line - intervals and segments. Isolated points are much less common. And even less often, it happens that the boundary of the solution (the end of the segment) coincides with the boundary of the range under consideration.
Consequently, if boundaries (the same “special cases”) are not included in the answer, then the areas to the left and right of these boundaries will almost certainly not be included in the answer. And vice versa: the border entered into the answer, which means that some areas around it will also be answers.
Keep this in mind when reviewing your solutions.
This article is devoted to techniques for solving various equations and inequalities containing
variable under the modulus sign.
If you come across an equation or inequality with a modulus in the exam, you can solve it by
without knowing any special methods and using only the module definition. Is it true,
This may take an hour and a half of precious exam time.
That’s why we want to tell you about techniques that simplify solving such problems.
First of all, let us remember that
Let's consider Various types equations with modulus. (We will move on to inequalities later.)
This is the simplest case. Let's solve the equation
There are only two numbers whose modules are equal to four. These are 4 and −4. Therefore the equation
is equivalent to the combination of two simple ones:
The second equation has no solutions. Solutions to the first: x = 0 and x = 5.
Answer: 0; 5.
Here we have to expand the module by definition. . . or think!
The equation splits into two cases, depending on the sign of the expression under the modulus.
In other words, it is equivalent to a combination of two systems:
Solution of the first system: . The second system has no solutions.
Answer: 1.
First case: x ≥ 3. Remove the module:
The number, being negative, does not satisfy the condition x ≥ 3 and therefore is not a root of the original equation.
Let's find out if it satisfies this condition number . To do this, we compose the difference and determine its sign:
This means that it is greater than three and therefore is the root of the original equation
Second case: x< 3. Снимаем модуль:
Number . greater than , and therefore does not satisfy the condition x< 3. Проверим :
Means, . is the root of the original equation.
Removing a module by definition? It's scary to even think about it, because the discriminant is not a perfect square. Let's better use the following consideration: an equation of the form |A| = B is equivalent to the combination of two systems:
Same thing, but a little different:
In other words, we solve two equations, A = B and A = −B, and then select roots that satisfy the condition B ≥ 0.
Let's get started. First we solve the first equation:
Then we solve the second equation:
Now in each case we check the sign of the right side:
Therefore, only and are suitable.
Let's solve the equation:
Since , it is convenient to make the replacement |x| = t. We get:
Answer: ±1.
We are talking about equations of the form |A| = |B|. This is a gift of fate. No module disclosures by definition! It's simple:
For example, consider the equation: . It is equivalent to the following set:
It remains to solve each of the equations of the set and write down the answer.
Let's solve the equation:
Let's not bother with each module separately and open it by definition - there will be too many options. There is a more rational way - the interval method.
The modulus expressions vanish at the points x = 1, x = 2, and x = 3. These points divide the number line into four intervals (intervals). Let's mark these points on the number line and place signs for each of the expressions under the modules on the resulting intervals. (The order of the signs coincides with the order of the corresponding modules in the equation.)
Thus, we need to consider four cases - when x is in each of the intervals.
Case 1: x ≥ 3. All modules are removed “with a plus”:
The resulting value x = 5 satisfies the condition x ≥ 3 and is therefore the root of the original equation.
Case 2: 2 ≤ x ≤ 3. The last module is now removed “with a minus”:
The resulting value of x is also suitable - it belongs to the interval under consideration.
Case 3: 1 ≤ x ≤ 2. The second and third modules are removed “with a minus”:
We have obtained the correct numerical equality for any x from the interval under consideration; they serve as solutions to this equation.
Case 4: x ≤ 1 ≤ 1. The second and third modules are removed “with a minus”:
Nothing new. We already know that x = 1 is a solution.
Answer: ∪ (5).
Let's solve the equation:
We start by opening the internal module.
1) x ≤ 3. We get:
The expression under the modulus vanishes at . This point belongs to the considered
between. Therefore, we have to analyze two subcases.
1.1) In this case we get:
This x value is not suitable because it does not belong to the interval under consideration.
1.2) . Then:
This x value is also not good.
So, for x ≤ 3 there are no solutions. Let's move on to the second case.
2) x ≥ 3. We have:
Here we are lucky: the expression x + 2 is positive in the interval under consideration! Therefore, there will no longer be any subcases: the module is removed “with a plus”:
This value of x is in the interval under consideration and is therefore the root of the original equation.
This is how all problems of this type are solved - we open the nested modules one by one, starting with the internal one.
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Modulus of numbers this number itself is called if it is non-negative, or the same number with the opposite sign if it is negative.
For example, the modulus of the number 6 is 6, and the modulus of the number -6 is also 6.
That is, the modulus of a number is understood as the absolute value, the absolute value of this number without taking into account its sign.
It is designated as follows: |6|, | X|, |A| etc.
(More details in the “Number module” section).
Equations with modulus.
Example 1 . Solve the equation|10 X - 5| = 15.
Solution.
According to the rule, the equation is equivalent to the combination of two equations:
10X - 5 = 15
10X - 5 = -15
We decide:
10X = 15 + 5 = 20
10X = -15 + 5 = -10
X = 20: 10
X = -10: 10
X = 2
X = -1
Answer: X 1 = 2, X 2 = -1.
Example 2 . Solve the equation|2 X + 1| = X + 2.
Solution.
Since the modulus is a non-negative number, then X+ 2 ≥ 0. Accordingly:
X ≥ -2.
Let's make two equations:
2X + 1 = X + 2
2X + 1 = -(X + 2)
We decide:
2X + 1 = X + 2
2X + 1 = -X - 2
2X - X = 2 - 1
2X + X = -2 - 1
X = 1
X = -1
Both numbers are greater than -2. So both are roots of the equation.
Answer: X 1 = -1, X 2 = 1.
Example 3
. Solve the equation
|X + 3| - 1
————— = 4
X - 1
Solution.
The equation makes sense if the denominator is not zero - that means if X≠ 1. Let's take this condition into account. Our first action is simple - we don’t just get rid of the fraction, but transform it so as to obtain the module in its pure form:
|X+ 3| - 1 = 4 · ( X - 1),
|X + 3| - 1 = 4X - 4,
|X + 3| = 4X - 4 + 1,
|X + 3| = 4X - 3.
Now we have only an expression under the modulus on the left side of the equation. Go ahead.
The modulus of a number is a non-negative number - that is, it must be greater than zero or equal to zero. Accordingly, we solve the inequality:
4X - 3 ≥ 0
4X ≥ 3
X ≥ 3/4
Thus, we have a second condition: the root of the equation must be at least 3/4.
In accordance with the rule, we compose a set of two equations and solve them:
X + 3 = 4X - 3
X + 3 = -(4X - 3)
X + 3 = 4X - 3
X + 3 = -4X + 3
X - 4X = -3 - 3
X + 4X = 3 - 3
X = 2
X = 0
We received two answers. Let's check whether they are roots of the original equation.
We had two conditions: the root of the equation cannot be equal to 1, and it must be at least 3/4. That is X ≠ 1, X≥ 3/4. Both of these conditions correspond to only one of the two answers received - the number 2. This means that only this is the root of the original equation.
Answer: X = 2.
Inequalities with modulus.
Example 1 . Solve inequality| X - 3| < 4
Solution.
The module rule states:
|A| = A, If A ≥ 0.
|A| = -A, If A < 0.
The module can have both non-negative and negative numbers. So we have to consider both cases: X- 3 ≥ 0 and X - 3 < 0.
1) When X- 3 ≥ 0 our original inequality remains as it is, only without the modulus sign:
X - 3 < 4.
2) When X - 3 < 0 в исходном неравенстве надо поставить знак минус перед всем подмодульным выражением:
-(X - 3) < 4.
Opening the brackets, we get:
-X + 3 < 4.
Thus, from these two conditions we came to the unification of two systems of inequalities:
X - 3 ≥ 0
X - 3 < 4
X - 3 < 0
-X + 3 < 4
Let's solve them:
X ≥ 3
X < 7
X < 3
X > -1
So, our answer is a union of two sets:
3 ≤ X < 7 U -1 < X < 3.
Determine the smallest and highest value. These are -1 and 7. Moreover X greater than -1 but less than 7.
Besides, X≥ 3. This means that the solution to the inequality is the entire set of numbers from -1 to 7, excluding these extreme numbers.
Answer: -1 < X < 7.
Or: X ∈ (-1; 7).
Add-ons.
1) There is a simpler and shorter way to solve our inequality - graphically. To do this, you need to draw a horizontal axis (Fig. 1).
Expression | X - 3| < 4 означает, что расстояние от точки X to point 3 is less than four units. We mark the number 3 on the axis and count 4 divisions to the left and to the right of it. On the left we will come to point -1, on the right - to point 7. Thus, the points X we just saw them without calculating them.
Moreover, according to the inequality condition, -1 and 7 themselves are not included in the set of solutions. Thus, we get the answer:
1 < X < 7.
2) But there is another solution that is even simpler graphic method. To do this, our inequality must be presented in the following form:
4 < X - 3 < 4.
After all, this is how it is according to the modulus rule. The non-negative number 4 and the similar negative number -4 are the boundaries for solving the inequality.
4 + 3 < X < 4 + 3
1 < X < 7.
Example 2 . Solve inequality| X - 2| ≥ 5
Solution.
This example is significantly different from the previous one. The left side is greater than 5 or equal to 5. C geometric point From the point of view, the solution to the inequality is all numbers that are at a distance of 5 units or more from point 2 (Fig. 2). The graph shows that these are all numbers that are less than or equal to -3 and greater than or equal to 7. This means that we have already received the answer.
Answer: -3 ≥ X ≥ 7.
Along the way, we solve the same inequality by rearranging the free term to the left and to the right with the opposite sign:
5 ≥ X - 2 ≥ 5
5 + 2 ≥ X ≥ 5 + 2
The answer is the same: -3 ≥ X ≥ 7.
Or: X ∈ [-3; 7]
The example is solved.
Example 3 . Solve inequality 6 X 2 - | X| - 2 ≤ 0
Solution.
Number X can be a positive number, negative number, or zero. Therefore, we need to take into account all three circumstances. As you know, they are taken into account in two inequalities: X≥ 0 and X < 0. При X≥ 0 we simply rewrite our original inequality as is, only without the modulus sign:
6x 2 - X - 2 ≤ 0.
Now about the second case: if X < 0. Модулем отрицательного числа является это же число с противоположным знаком. То есть пишем число под модулем с обратным знаком и опять же освобождаемся от знака модуля:
6X 2 - (-X) - 2 ≤ 0.
Expanding the brackets:
6X 2 + X - 2 ≤ 0.
Thus, we received two systems of equations:
6X 2 - X - 2 ≤ 0
X ≥ 0
6X 2 + X - 2 ≤ 0
X < 0
We need to solve inequalities in systems - and this means we need to find the roots of two quadratic equations. To do this, we equate the left-hand sides of the inequalities to zero.
Let's start with the first one:
6X 2 - X - 2 = 0.
How to solve a quadratic equation - see section “ Quadratic equation" We will immediately name the answer:
X 1 = -1/2, x 2 = 2/3.
From the first system of inequalities we obtain that the solution to the original inequality is the entire set of numbers from -1/2 to 2/3. We write the union of solutions at X ≥ 0:
[-1/2; 2/3].
Now let's solve the second quadratic equation:
6X 2 + X - 2 = 0.
Its roots:
X 1 = -2/3, X 2 = 1/2.
Conclusion: when X < 0 корнями исходного неравенства являются также все числа от -2/3 до 1/2.
Let's combine the two answers and get the final answer: the solution is the entire set of numbers from -2/3 to 2/3, including these extreme numbers.
Answer: -2/3 ≤ X ≤ 2/3.
Or: X ∈ [-2/3; 2/3].
