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Precalculus for Team-Based Inquiry Learning: PREVIEW Edition — Instructor Version

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Section 1.4 Absolute Value Equations and Inequalities (EQ4)

Subsection 1.4.1 Activities

Remark 1.4.1.

\(\lvert x \rvert\text{,}\)\(x\text{.}\)\(x\)\(\lvert x \rvert=x\text{.}\)\(x\)\(\lvert x \rvert=-x\text{.}\)

Activity 1.4.2.

Let’s consider how to solve an equation when an absolute value is involved.
(a)
Which values are solutions to the absolute value equation \(\lvert x \rvert = 2\text{?}\)
  1. \(\displaystyle x=2\)
  2. \(\displaystyle x=0\)
  3. \(\displaystyle x=-1\)
  4. \(\displaystyle x=-2\)
Answer.
A and D
(b)
Which values are solutions to the absolute value equation \(\lvert x-7 \rvert = 2\text{?}\)
  1. \(\displaystyle x=9\)
  2. \(\displaystyle x=7\)
  3. \(\displaystyle x=5\)
  4. \(\displaystyle x=-9\)
Answer.
A and C
(c)
Which values are solutions to the absolute value equation \(3\lvert x-7 \rvert +5= 11\text{?}\) It may be helpful to rewrite the equation to isolate the absolute value.
  1. \(\displaystyle x=7\)
  2. \(\displaystyle x=-9\)
  3. \(\displaystyle x=5\)
  4. \(\displaystyle x=9\)
Answer.
C and D

Activity 1.4.3.

Absolute value represents the distance a value is from 0 on the number line. So, \(\lvert x-7 \rvert = 2\) means that the expression \(x-7\) is \(2\) units away from \(0\text{.}\)
(a)
What values on the number line could \(x-7 \) equal?
  1. \(\displaystyle x=-7\)
  2. \(\displaystyle x=-2\)
  3. \(\displaystyle x=0\)
  4. \(\displaystyle x=2\)
  5. \(\displaystyle x=7\)
Answer.
B and D
(b)
This gives us two separate equations to solve. What are those two equations?
  1. \(\displaystyle x-7=-7\)
  2. \(\displaystyle x-7=-2\)
  3. \(\displaystyle x-7=0\)
  4. \(\displaystyle x-7=2\)
  5. \(\displaystyle x-7=7\)
Answer.
B and D
(c)
Solve each equation for \(x\text{.}\)
Answer.
\(x=-5\) and \(x=9\)

Remark 1.4.4.

\(c \gt 0\text{,}\)
\begin{equation*} \lvert ax+b \rvert = c \end{equation*}
\begin{equation*} ax+b =c \quad \text{and} \quad ax+b=-c \end{equation*}

Activity 1.4.5.

Solve the following absolute value equations.
(a)
\(\lvert 3x+4 \rvert = 10\)
  1. \(\displaystyle \{-2, 2\}\)
  2. \(\displaystyle \left\{-\dfrac{14}{3}, 2\right\}\)
  3. \(\displaystyle \{-10, 10\}\)
  4. No solution
Answer.
B
(b)
\(3\lvert x-7 \rvert+5 = 11\)
  1. \(\displaystyle \{-2, 2\}\)
  2. \(\displaystyle \{-9, 9\}\)
  3. \(\displaystyle \{5, 9\}\)
  4. No solution
Answer.
C
(c)
\(2\lvert x+1 \rvert+8 = 4\)
  1. \(\displaystyle \{-4, 4\}\)
  2. \(\displaystyle \{-6, 6\}\)
  3. \(\displaystyle \{5, 7\}\)
  4. No solution
Answer.
D

Remark 1.4.6.

Activity 1.4.7.

Just as with linear equations and inequalities, we can consider absolute value inequalities from equations.
(a)
Which values are solutions to the absolute value inequality \(\lvert x-7 \rvert \le 2\text{?}\)
  1. \(\displaystyle x=9\)
  2. \(\displaystyle x=7\)
  3. \(\displaystyle x=5\)
  4. \(\displaystyle x=-9\)
Answer.
A, B and C
(b)
Rewrite the absolute value inequality \(\lvert x-7 \rvert \le 2\) as a compound inequality.
  1. \(\displaystyle 0 \le x-7 \le 2\)
  2. \(\displaystyle -2 \le x-7 \le 2\)
  3. \(\displaystyle -2 \le x-7 \le 0\)
  4. \(\displaystyle 2 \le x \le 7\)
Answer.
B
(c)
Solve the compound inequality that is equivalent to \(\lvert x-7 \rvert \le 2\) found in part (b). Write the solution in interval notation.
  1. \(\displaystyle [7,9]\)
  2. \(\displaystyle [5,9]\)
  3. \(\displaystyle [5,7]\)
  4. \(\displaystyle [2,7]\)
Answer.
B
(d)
Draw the solution to \(\lvert x-7 \rvert \le 2\) on the number line.
Answer.
B

Activity 1.4.8.

Now let’s consider another type of absolute value inequality.
(a)
Which values are solutions to the absolute value inequality \(\lvert x-7 \rvert \ge 2\text{?}\)
  1. \(\displaystyle x=9\)
  2. \(\displaystyle x=7\)
  3. \(\displaystyle x=5\)
  4. \(\displaystyle x=-9\)
Answer.
A, C and D
(b)
Which two of the following inequalities are equivalent to \(\lvert x-7 \rvert \ge 2\text{.}\)
  1. \(\displaystyle x-7 \le 2 \)
  2. \(\displaystyle x-7 \le -2\)
  3. \(\displaystyle x-7 \ge 2\)
  4. \(\displaystyle x-7 \ge -2\)
Answer.
B and C
(c)
Solve the two inequalities found in part (b). Write the solution in interval notation and graph on the number line.
  1. \((-\infty,7] \cup [9,\infty)\)
  2. \((-\infty,5] \cup [9,\infty)\)
  3. \((-\infty,5] \cup [7,\infty)\)
  4. \((-\infty,2] \cup [7,\infty)\)
Answer.
B

Definition 1.4.9.

When solving an absolute value inequality, rewrite it as compound inequalities. Assume \(k\) is positive. \(\lvert x \rvert \lt k \text{ becomes } -k \lt x \lt k\text{.}\) \(\lvert x \rvert \gt k \text{ becomes } x\gt k \text{ or } x\lt-k\text{.}\)

Activity 1.4.10.

Solve the following absolute value inequalities. Write your solution in interval notation and graph on a number line.
(a)
\(\lvert 3x+4 \rvert \lt 10\)
Answer.
\(-\dfrac{14}{3} \lt x \lt 2\text{,}\) \(\left( -\dfrac{14}{3}, 2 \right)\)
(b)
\(3\lvert x-7 \rvert+5 \gt 11\)
Answer.
\(x \lt 5\) and \(x \gt 9\text{,}\) \((-\infty, 5) \cup (9, \infty)\)

Exercises 1.4.2 Exercises

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