Algebra 1 : How to solve absolute value equations

Study concepts, example questions & explanations for Algebra 1

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Example Questions

Example Question #1 : How To Solve Absolute Value Equations

Solve the absolute value equation:

\displaystyle \left | \frac{x-4}{3} \right| = \left | x +2 \right|

Possible Answers:

\displaystyle \left \{-5 \right \}

\displaystyle \left \{-5, -\frac{1}{2} \right \}

\displaystyle \small \left \{-5, -1 \right\}

 (no solution)

Correct answer:

\displaystyle \left \{-5, -\frac{1}{2} \right \}

Explanation:

An equation that equates two absolute value functions allows us to choose one of the absolute value functions and treat it as the constant. We then separate the equation into the "positive" version, \displaystyle \tiny \small \frac{x-4}{3}=x+2 , and the "negative" version,\displaystyle \tiny \small \frac{x-4}{3} = -(x+2). Solving each equation, we obtain the solutions, \displaystyle -5 and \displaystyle \tiny \tiny -\frac{1}{2}, respectively.  

Example Question #1 : How To Solve Absolute Value Equations

Solve for x.

\displaystyle |x+3|=4

Possible Answers:

x = 4, 7

x = 1, 7

x = –3, 4

x = –7, 1

No solution

Correct answer:

x = –7, 1

Explanation:

First, split \displaystyle |x+3|=4 into two possible scenarios according to the absolute value.

\displaystyle x+3=4

\displaystyle x+3=-4

Looking at \displaystyle x+3=4, we can solve for x by subtracting 3 from both sides, so that we get x = 1.

Looking at \displaystyle x+3=-4, we can solve for x by subtracting 3 from both sides, so that we get x = –7.

So therefore, the solution is x = –7, 1.

Example Question #1 : How To Solve Absolute Value Equations

Find the solution to x for |x – 3| = 2.

Possible Answers:

2, 5

2, 4

1, 5

0

1, 4

Correct answer:

1, 5

Explanation:

|x – 3| = 2 means that it can be separated into x – 3 = 2 and x – 3 = –2.

So both x = 5 and x = 1 work.

x – 3 = 2 Add 3 to both sides to get x = 5

x – 3 = –2 Add 3 to both sides to get x = 1

Example Question #2 : How To Solve Absolute Value Equations

Solve for x:

\displaystyle \small \left | x+2 \right |=12

Possible Answers:

\displaystyle \small x=-14 or \displaystyle \small x=10

\displaystyle \small x=14 or \displaystyle \small x=-10

\displaystyle \small x=-10

\displaystyle \small x=10

Correct answer:

\displaystyle \small x=-14 or \displaystyle \small x=10

Explanation:

Because of the absolute value signs,

\displaystyle \small x+2=12 or \displaystyle \small x+2=-12

Subtract 2 from both sides of both equations:

\displaystyle \small x+2-2=12-2 or \displaystyle \small x+2-2=-12-2

\displaystyle \small x=10 or \displaystyle \small x=-14

Example Question #4 : How To Solve Absolute Value Equations

Solve for \displaystyle x:

\displaystyle |x-4|=\frac{5}{8}

Possible Answers:

\displaystyle x=4\frac{5}{8}, -3\frac{5}{8}

\displaystyle x=4\frac{3}{8}, -3\frac{3}{8}

\displaystyle x=4\frac{5}{8}

\displaystyle x=4\frac{5}{8}, -4\frac{3}{8}

\displaystyle x=4\frac{5}{8}, -3\frac{3}{8}

Correct answer:

\displaystyle x=4\frac{5}{8}, -3\frac{3}{8}

Explanation:

\displaystyle |x-4|=\frac{5}{8}

There are two answers to this problem:

\displaystyle x-4=\frac{5}{8}

\displaystyle x=\frac{5}{8}+4=4\frac{5}{8}

and

\displaystyle -(x-4)=\frac{5}{8}

\displaystyle 4-x=\frac{5}{8}

\displaystyle -x=\frac{5}{8}-4=-3\frac{3}{8}

\displaystyle x=-3\frac{3}{8}

Example Question #1 : How To Solve Absolute Value Equations

If \displaystyle x = 2, evaluate \displaystyle \left | -4x + 3\right | \left | 3x-14\right |.

Possible Answers:

\displaystyle -40

\displaystyle -3

\displaystyle 40

\displaystyle 13

\displaystyle 220

Correct answer:

\displaystyle 40

Explanation:

An absolute value expression differs from a normal expression only in its sign. Instead of being a positive or negative quantity, an absolute value represents a scalar distance from zero, so it does not have a sign. For example, \displaystyle \left | -2\right | is the same as \displaystyle \left | 2\right | because both represent a value 2 units away from zero. In this problem, \displaystyle \left | -4x + 3\right | equals \displaystyle \left | -5 \right |, or 5. \displaystyle \left | 3x - 14\right | equals 8. The final answer is \displaystyle (5)(8) or 40.

Example Question #1 : How To Solve Absolute Value Equations

Possible Answers:

\displaystyle -6, 2

\displaystyle 2, 6

\displaystyle 0, 16

\displaystyle -6, 6

\displaystyle -2, 6

Correct answer:

\displaystyle -2, 6

Explanation:

\displaystyle 2a-4=8\; \; \; \; \; \; \; \; \; \; \; \; 2a-4=-8

\displaystyle 2a=12\;\;\;\;\;\;\;\;\;\;\; \; \; \; \; \; \; 2a=-4

\displaystyle a=6\; \; \; \;\;\:\;\;\;\;\;\;\;\;\;\;\;\;\;\;a=-2

Example Question #6 : How To Solve Absolute Value Equations

What is the solution set of this equation?

\displaystyle \left | 3x-7\right |+7 = -6

Possible Answers:

\displaystyle \mathbb{R}

Correct answer:

Explanation:

\displaystyle \left | 3x-7\right |+7 = -6

To find a solution, subtract \displaystyle 7 first to isolate the absolute value expression.

\displaystyle \left | 3x-7\right | = -13

There is no value of \displaystyle x that makes this true, as no number has a negative absolute value. The equation has no solution.

Example Question #1 : How To Solve Absolute Value Equations

 \displaystyle Find \; all \;solutions\; for\;x \; in \;\left | 5x+3\right |-3=0

Possible Answers:

\displaystyle 0 

\displaystyle 0,\, \frac{-6}{5}

\displaystyle 0,3,\, \frac{-6}{5}

\displaystyle \frac{-6}{5}

Correct answer:

\displaystyle 0,\, \frac{-6}{5}

Explanation:
\(\displaystyle \left | 5x+3\right |-3=0\)
\(\displaystyle \left | 5x+3\right |=3\)

\(\displaystyle 5x+3 = 3 \;or\; 5x+3 = -3\)

\(\displaystyle First \;equation: 5x+3=3\)
\(\displaystyle 5x=0\)
\(\displaystyle x=0\)


\(\displaystyle Second\;equation:5x+3 = -3\)

\(\displaystyle 5x=-6\)
\(\displaystyle x = -6/5\)

\(\displaystyle Therefore, \;x =0, -6/5\)

Example Question #8 : How To Solve Absolute Value Equations

Solve for \displaystyle x

\displaystyle \left | 4x+19 \right | = 31

Possible Answers:

\displaystyle x = 3 or \displaystyle x = 12.5 

\displaystyle x = -3 or \displaystyle x=3

\displaystyle x = -12.5 or \displaystyle x = 3

\displaystyle x = 3

\displaystyle x = -3

Correct answer:

\displaystyle x = -12.5 or \displaystyle x = 3

Explanation:

\displaystyle \left | 4x+19 \right | = 31 can be rewritten as the compound statement:

\displaystyle 4x+19= - 31  or  \displaystyle 4x+19= 31

 

Solve each separately to obtain the solution set:

 

\displaystyle 4x+19= - 31

\displaystyle 4x+19 - 19 = -31- 19

\displaystyle 4x = -50

\displaystyle 4x \div 4= -50 \div 4

\displaystyle x = -12.5

 

\displaystyle 4x+19= 31

\displaystyle 4x+19 - 19 = 31- 19

\displaystyle 4x = 12

\displaystyle 4x \div 4= 12 \div 4

\displaystyle x = 3

 

So either \displaystyle x = -12.5 or \displaystyle x = 3

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