Question

Solve 7|3f + 4| = 91 for f. A. –29 or 312⁄3 B. 3 or –17⁄3 C. –3 or 17⁄3 D. –312⁄3 or 29

Answer 1

B

Explanation:

Given

7 | 3f + 4 | = 91 ← divide both sides by 7

| 3f + 4 | = 13

The absolute value function always returns a positive value, but the expression inside can be positive or negative. Thus there are 2 possible solutions.

Solve 3f + 4 = 13 → 3f = 13 - 4 = 9 ⇒ f = 3

Solve 3f + 4 = - 13 ⇒ 3f = - 13 - 4 = - 17 ⇒ f = -
`(17)/(3)`

As a check

Substitute these values into the left side of the equation and if equal to the right side then they are the solutions.

f = 3 : 7| 9 + 4| = 7| 13| = 7 × 13 = 91 ← True

f = -
`(17)/(3)`

7 | - 17 + 4| = 7| - 13| = 7 × 13 = 91 ← True

Hence the correct solutions are B