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Equations · Equations

Absolute Value Equations

Before you start, make sure you know: First-degree equations (Secondary Year 3)
Before you start, make sure you know: Second-degree equations (High School Year 1 (semesters 1 and 2))

At a street market (tianguis), the scale at the produce stand should give bags of $1{,}000$ g, but it is allowed to be off by up to $20$ g, either above or below. What are the limit weights of an acceptable bag? To answer, it helps to know that an absolute value equation hides two equations at once. At the end of the page we come back to the scale.

Definition and the basic case $|ax+b|=k$

Number line with the numbers -5 and 13, which are at the same distance 9 from the number 4. 99 −5134 |x − 4| = 9: two numbers at distance 9 from 4
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Solve the following linear equation with absolute value: $|3x - 5| = 7$

Solve the following linear equation with absolute value:

$|3x - 5| = 7$

Solve
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Isolate the absolute value and find the solutions to the equation: $2|x + 4| - 10 = 0$

Isolate the absolute value and find the solutions to the equation:

$2|x + 4| - 10 = 0$

Solve
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Solve the following equation with fractional coefficients: $\left| \frac{2x}{3} - 1 \right| = 5$

Solve the following equation with fractional coefficients:

$\left| \frac{2x}{3} - 1 \right| = 5$

Solve

Distance on the real number line

Setting up a distance problem means translating "at distance $d$ from $a$" as $|x-a|=d$. Two points always come out, one on each side of $a$: $x=a+d$ and $x=a-d$.

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Find the numbers whose distance to the number $4$ on the number line is exactly $9$. Set up and solve the corresponding absolute value equation.

Find the numbers whose distance to the number $4$ on the number line is exactly $9$. Set up and solve the corresponding absolute value equation.

Solve

No solution and two equal absolute values

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Solve the equation by considering that the arguments of the absolute values are equal or opposite: $|2x - 1| = |x + 3|$

Solve the equation by considering that the arguments of the absolute values are equal or opposite:

$|2x - 1| = |x + 3|$

Solve
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Analyse and solve the following equation. Justify your answer: $|5x + 2| = -4$

Analyse and solve the following equation. Justify your answer:

$|5x + 2| = -4$

Solve
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Solve the following equation by equating the absolute value of a quadratic expression to the absolute value of a linear expression: $|x^2 - 5x + 4| = |2x - 6|$

Solve the following equation by equating the absolute value of a quadratic expression to the absolute value of a linear expression:

$|x^2 - 5x + 4| = |2x - 6|$

Solve

Absolute value of quadratic expressions

The graph of $|x^2-5x|$ is the parabola with the part below the $x$-axis "folded" upward. The line $y=6$ cuts it at four points: those are the solutions of $|x^2-5x|=6$.

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Find the values of $x$ that satisfy the following quadratic equation with absolute value: $|x^2 - 9| = 0$

Find the values of $x$ that satisfy the following quadratic equation with absolute value:

$|x^2 - 9| = 0$

Solve
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Find all the real solutions to the equation: $|x^2 - 5x| = 6$

Find all the real solutions to the equation:

$|x^2 - 5x| = 6$

Solve

When the other side has the unknown

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Solve the following equation and check the validity of the solutions obtained, discarding any that do not make sense: $|x - 5| = 2x - 1$

Solve the following equation and check the validity of the solutions obtained, discarding any that do not make sense:

$|x - 5| = 2x - 1$

Solve
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Find the values of $x$ that satisfy the following equation, checking the solutions in the original equation: $|x^2 - 4x| = 3x - 6$

Find the values of $x$ that satisfy the following equation, checking the solutions in the original equation:

$|x^2 - 4x| = 3x - 6$

Solve
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A particle oscillates around an equilibrium point. Its position $x$ at certain critical moments satisfies the equation $|x^2 - 10| = 3x$. Find the exact positions $x$…

A particle oscillates around an equilibrium point. Its position $x$ at certain critical moments satisfies the equation $|x^2 - 10| = 3x$. Find the exact positions $x$ where this occurs, making sure to discard invalid solutions.

Solve

Quotients with absolute value

When there is a fraction inside the absolute value, it is solved the same way with one extra step: the denominator cannot equal zero. The values that make it zero are excluded from the start.

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Solve the following rational equation with absolute value: $\left| \frac{x+2}{x-1} \right| = 3$

Solve the following rational equation with absolute value:

$\left| \frac{x+2}{x-1} \right| = 3$

Solve
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Solve the following rational equation involving an absolute value. Pay special attention to the values that make the denominator zero:…

Solve the following rational equation involving an absolute value. Pay special attention to the values that make the denominator zero:

$\left| \frac{x^2 - x - 2}{x^2 + x} \right| = 1$

Solve

Equations that mix $|x|$ with $x^2$

If $x^2$ and $|x|$ both appear in the equation, there are two paths.

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Find the solutions to the following equation by making a suitable substitution or distinguishing cases according to the sign of $x$: $x^2 - 4|x| - 5 = 0$

Find the solutions to the following equation by making a suitable substitution or distinguishing cases according to the sign of $x$:

$x^2 - 4|x| - 5 = 0$

Solve
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Find all the real solutions to the combined equation: $x^2 - 2|x - 1| = 2$

Find all the real solutions to the combined equation:

$x^2 - 2|x - 1| = 2$

Solve

Nested absolute values and intervals

The number line divided into three intervals by the points -3 and 2, where the arguments of the equation |x−2| + |x+3| = 7 vanish. |x−2| + |x+3| = 7 −32 x < −3−3 ≤ x ≤ 2x > 2 −2x − 1 = 75 = 72x + 1 = 7 x = −4 ✓impossible ✗x = 3 ✓
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Solve the following equation with nested absolute values: $||2x - 1| - 4| = 5$

Solve the following equation with nested absolute values:

$||2x - 1| - 4| = 5$

Solve
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Find the solutions to the equation by dividing the number line into intervals according to the roots of the arguments of the absolute values: $|x - 2| + |x + 3| = 7$

Find the solutions to the equation by dividing the number line into intervals according to the roots of the arguments of the absolute values:

$|x - 2| + |x + 3| = 7$

Solve
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A robotic arm moves along a linear rail. Its distance to two beacons located at positions $x=0$ and $x=10$ must satisfy the constraint given by the equation…

A robotic arm moves along a linear rail. Its distance to two beacons located at positions $x=0$ and $x=10$ must satisfy the constraint given by the equation $2|x| - |x - 10| = 5$. Find the exact $x$ coordinates where the robot can stop.

Solve

How many solutions? The parameter $k$

With the graph you can count solutions without solving. The solutions of $|f(x)|=k$ are the crossings of the graph of $y=|f(x)|$ with the horizontal line $y=k$. Move $k$ in the figure.

The graph of $|x^2-4x+3|$ has a "W" shape: the original parabola has its vertex at $(2,-1)$ and crosses the axis at $x=1$ and $x=3$, and the lower part is reflected, forming a peak at $(2,1)$.

Value of $k$ Solutions
$k<0$ none
$k=0$ two ($x=1$, $x=3$)
$0 four
$k=1$ three (right at the peak)
$k>1$ two
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Determine the value of the parameter $k$ for which the equation $|x^2 - 4x + 3| = k$ has exactly three distinct real solutions. In the figure you have the graph of…

Determine the value of the parameter $k$ for which the equation $|x^2 - 4x + 3| = k$ has exactly three distinct real solutions.

In the figure you have the graph of $f(x)=|x^2-4x+3|$. Use it to reason how many solutions the equation has depending on the height of the horizontal line $y=k$, and justify the value of $k$ with calculations.

Graph of the function f(x) = |x² − 4x + 3| with the axes marked in units x y 1 2 3 4 1 2 3 f(x)
Solve

Applications: tolerances and margins

Number line with the nominal weight of 1000 grams and the acceptable limits 980 and 1020 grams, at distance 20 from the center. |m − 1000| = 20 2020 98010201000 m is within 20 g of 1000 g

A tolerance is the maximum difference allowed from an ideal value, and it is written with absolute value: $|\text{value}-\text{ideal}|=\text{tolerance}$ gives the two limits.

Back to the start: the scale at the tianguis. $|m-1000|=20$ gives $m=1{,}020$ g or $m=980$ g. A bag is acceptable between $980$ g and $1{,}020$ g.

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A machine cuts steel cables for a suspension bridge. The length $L$ (in meters) of a cable is considered acceptable if it satisfies the tolerance equation…

A machine cuts steel cables for a suspension bridge. The length $L$ (in meters) of a cable is considered acceptable if it satisfies the tolerance equation $|2L - 150| = 1.5$. Calculate the maximum and minimum acceptable lengths for the cable.

Solve
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In an economic model, the deviation of the price $P$ of a share from its benchmark value of $2\,400$ pesos is proportional to the price itself, modeled by the equation…

In an economic model, the deviation of the price $P$ of a share from its benchmark value of $2\,400$ pesos is proportional to the price itself, modeled by the equation $|P - 2\,400| = \frac{P}{5}$. Calculate the exact prices that satisfy this condition.

Solve
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During a chemistry experiment, the margin of error in measuring the pH of a buffer solution is given by the equation $|2p - 14| = p - 4$, where $p$ is the pH value.…

During a chemistry experiment, the margin of error in measuring the pH of a buffer solution is given by the equation $|2p - 14| = p - 4$, where $p$ is the pH value. Calculate the possible values of $p$ and discard any that lack physical or mathematical meaning in this equation.

Solve

And the support of a camping tent, with a beam at a certain height, is solved with intervals just like in the previous section:

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The entrance profile of a modern-design tent is modeled by the equation $y = |x - 4| + |x - 8|$, where $x$ and $y$ are in meters. To place a horizontal support beam at a…

The entrance profile of a modern-design tent is modeled by the equation $y = |x - 4| + |x - 8|$, where $x$ and $y$ are in meters. To place a horizontal support beam at a height of $y = 6$ meters, solve the equation $|x - 4| + |x - 8| = 6$ to find the $x$ coordinates of the ends of the support.

Solve

Summary

See all the exercises in this section