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.
Both questions are based on the following function.
f (x) = x2 – 4x
- What is the value of the function when x = –4?
- For what value of x does f (x) = 12?
B. 2 and –6
C. 6 and –2
D. 0 and 6
Answers and Explanations
- The correct answer is D.
Rewrite the function by substituting in x = –4:
f (–4) = (–4)2 – 4(–4)
= 16 + 16
Hence, Choice (D) is the correct answer.
- 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
(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.