In this lesson, we'll learn how to solve equations that contain absolute value expressions.

For example, let's consider the following equation:

|x+1| = 4

For this equation to be true, either the positive or negative of the expression inside the absolute value must equal the right-hand side. That is,

x+1 = 4 \qquad\qquad \text{ or } \qquad\qquad -(x+1)=4.

So, let's solve these two equations:

  • First, we solve the equation x+1 = 4{:} \begin{align*} x + 1 &=4 \\[5pt] x + 1 - 1 &=4 - 1 \\[5pt] x &={\color{blue}3} \\[5pt] \end{align*}
  • Then, we solve the equation -(x+1) = 4{:} \begin{align*} -(x+1) &= 4 \\[5pt] -x-1 &= 4 \\[5pt] -x-1+1 &= 4+1 \\[5pt] -x &=5 \\[5pt] x &={\color{red}{-5}} \end{align*}

Therefore, the solutions to our original absolute value equation are x={\color{blue}3} and x={\color{red}{-5}}.

FLAG

So, we've seen that the solutions to the equation

|x+1| = 4

are x={\color{blue}3} and x={\color{red}{-5}}.

We can verify that these solutions are correct by substituting them back into the original equation and checking we get a true statement:

  • Substituting x={\color{blue}3} into our equation, we get \begin{align} |x+1| &= 4 \\[5pt] |{\color{blue}3}+1| &= 4 \\[5pt] |4| &= 4 \\[5pt] 4 &= 4.\quad{\color{green}{\checkmark}} \end{align} Therefore, x={\color{blue}3} is indeed a solution to the absolute value equation.

  • Substituting x={\color{red}-5} into our equation, we get \begin{align} |x+1| &= 4 \\[5pt] |{\color{red}-5}+1| &= 4 \\[5pt] |-4| &= 4 \\[5pt] 4 &= 4.\quad{\color{green}{\checkmark}} \end{align} Therefore, x={\color{red}-5} is indeed a solution to the absolute value equation.

Therefore, x={\color{blue}3} and x={\color{red}{-5}} are both solutions to our absolute value equation.

FLAG

Solve the equation \left\vert 7x \right\vert = 21.

EXPLANATION

Either the positive or negative of the expression inside the absolute value must be equal to the right-hand side of the equation.

So, we set up the two corresponding equations and solve them independently:

  • If |7x| = 7x, we obtain: \begin{align} |7x| & = 21 \\[5pt] 7x & = 21 \\[5pt] \dfrac{7x}{7} & = \dfrac{21}7 \\[5pt] x &=3 \end{align}

  • If |7x| = -7x, we obtain: \begin{align} |7x| & = 21 \\[5pt] -7x & = 21 \\[5pt] \dfrac{-7x}{-7} & = \dfrac{21}{-7} \\[5pt] x &=-3 \end{align}

Therefore, the solutions are x=3 and x=-3. We can write this more compactly as x = \pm 3.

FLAG

If $|z-2| = 3,$ then $z =$

a
$-5$ only
b
$-5$ or $ -1$
c
$-1$ or $5$
d
$1$ or $5$
e
$1$ only

Written in ascending order, the solutions to $|7z| = 42$ are $z =$

a
b
c
d
e

Written in ascending order, the solutions to $\left|\dfrac t8\right| = 9$ are $t =$

a
b
c
d
e

Solve the equation | 2x+9 | = 15.

EXPLANATION

Either the positive or negative of the expression inside the absolute value must be equal to the right-hand side of the equation.

So, we set up the two corresponding equations and solve them independently:

  • If |2x+9| = 2x+9, we obtain: \begin{align} |2x+9| &= 15 \\[5pt] 2x+9 &= 15 \\[5pt] 2x &= 6 \\[5pt] \dfrac{2x}2 &= \dfrac{6}2\\[5pt] x &= 3 \end{align}

  • If |2x+9| = -(2x+9), we obtain: \begin{align} |2x+9| &= 15 \\[5pt] -(2x+9) &= 15 \\[5pt] -2x-9 &= 15 \\[5pt] -2x &= 24 \\[5pt] \dfrac{-2x}{-2} &= \dfrac{24}{-2}\\[5pt] x &= -12 \end{align}

Therefore, the solutions are x=-12 and x=3.

FLAG

Written in ascending order, the solutions to $|3z+7| = 5$ are $z =$

a
b
c
d
e

Written in ascending order, the solutions to $|3x-4| = 1$ are $x =$

a
b
c
d
e

Solve the equation |5-8y| = 11.

EXPLANATION

Either the positive or negative of the expression inside the absolute value must be equal to the right-hand side of the equation.

So, we set up the two corresponding equations and solve them independently:

  • If |5-8y| = 5-8y, we obtain: \begin{align} |5-8y| &= 11 \\[5pt] 5-8y &= 11 \\[5pt] -8y &= 6 \\[5pt] \dfrac{-8y}{-8} &= \dfrac{6}{-8}\\[5pt] y &= -\dfrac34 \end{align}

  • If |5-8y| = -(5-8y), we obtain: \begin{align} |5-8y| &= 11 \\[5pt] -(5-8y) &= 11 \\[5pt] -5+8y &= 11 \\[5pt] 8y &= 16 \\[5pt] \dfrac{8y}{8} &= \dfrac{16}{8}\\[5pt] y &= 2 \end{align}

Therefore, the solutions are y=-\dfrac34 and y=2.

FLAG

Written in ascending order, the solutions to $|2-k| = 9$ are $k =$

a
b
c
d
e

If $|4-3x| = 14,$ then $x=$

a
$\pm 6$
b
$\dfrac{10}3$ only
c
$-6$ only
d
$-6$ or $\dfrac{10}3$
e
$-\dfrac{10}3$ or $6$

Solve the equation \left| \dfrac{7}{2}z-2 \right| = 3.

EXPLANATION

Either the positive or negative of the expression inside the absolute value must be equal to the right-hand side of the equation.

So, we set up the two corresponding equations and solve them independently:

  • If \left| \dfrac{7}{2}z-2 \right| = \dfrac{7}{2}z-2, we obtain: \begin{align*} \left| \dfrac{7}{2}z-2 \right| &= 3 \\[5pt] \dfrac{7}{2}z-2 &= 3 \\[5pt] \dfrac{7}{2}z &= 5 \end{align*} Multiplying both sides of the equation by \dfrac{2}{7} gives \begin{align*} \dfrac{7}{2}z & = 5 \\[5pt] \left(\dfrac{2}{7}\right) \cdot \left(\dfrac{7}{2}z\right) & = \left(\dfrac{2}{7}\right) \cdot (5) \\[5pt] \left(\cancel{\dfrac{2}{7}}\right) \cdot \left(\cancel{\dfrac{7}{2}}z\right) & = \dfrac{10}{7}\\[5pt] z & = \dfrac{10}{7}. \end{align*}

  • If \left| \dfrac{7}{2}z-2 \right| = -\left(\dfrac{7}{2}z-2\right), we obtain: \begin{align*} \left| \dfrac{7}{2}z-2 \right| & = 3 \\[5pt] -\left( \dfrac{7}{2}z-2 \right) & = 3 \\[5pt] -\dfrac{7}{2}z + 2 &= 3 \\[5pt] -\dfrac{7}{2}z &= 1 \end{align*} Multiplying both sides of the equation by -\dfrac{2}{7} gives \begin{align*} -\dfrac{7}{2}z & = 1 \\[5pt] \left(-\dfrac{2}{7}\right) \cdot \left(-\dfrac{7}{2}z \right) & = \left(-\dfrac{2}{7}\right) \cdot (1) \\[5pt] \left(\cancel{-\dfrac{2}{7}}\right) \cdot \left(\cancel{-\dfrac{7}{2}}z \right) & = -\dfrac{2}{7} \\[5pt] z & = -\dfrac{2}{7}. \end{align*}

Therefore, the solutions are z = -\dfrac{2}{7} and z = \dfrac{10}{7}.

FLAG

Written in ascending order, the solutions to $\left| \dfrac{1}{2}x + 3 \right| = 5$ are $x =$

a
b
c
d
e

If $\left\vert 4-\dfrac32x \right\vert = 2,$ then $x=$

a
$\dfrac43$ or $4$
b
$4$ only
c
$-2$ only
d
$-2$ or $\dfrac43$
e
$-2$ or $4$
Flag Content
Did you notice an error, or do you simply believe that something could be improved? Please explain below.
SUBMIT
CANCEL