GED Math Extra Prep: Functions

Function problems on the GED Math test can often be solved using substitution. In the following practice questions, you're first asked to find the value of a function f (x), and then the value of x.

Practice Questions

Both questions are based on the following function.

f (x) = x² – 4x

1. What is the value of the function when x = –4? A. –8 B. 0 C. 16 D. 32
2. For what value of x does f (x) = 12? A. 96 B. 2 and –6 C. 6 and –2 D. 0 and 6

Answers and Explanations

1. The correct answer is D.

   Rewrite the function by substituting in x = –4:

   f (–4) = (–4)² – 4(–4)

   = 16 + 16

   = 32

   Hence, Choice (D) is the correct answer.

2. The correct answer is C.

   Substitute f (x) = 12 into the equation:

   12 = x² – 4x

   Subtract 12 from both sides to set the equation equal to zero:

   0 = x² – 4x – 12

   Then factor:

   (x – 6)(x + 2) = 0

   Set each set of parentheses equal to zero and solve for x. You get x = 6 and x = –2. Hence, Choice (C) is the correct answer.