GED Test
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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.

About This Article

This article is from the book:

About the book author:

Stuart Donnelly, PhD, earned his doctorate in mathe-matics from Oxford University at the age of 25. Since then, he has established successful tutoring services in both Hong Kong and the United States and is considered by leading educators to be one of the most experienced and qualified private tutors in the country.

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