By Consumer Dummies

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.