Math · Intermediate Algebra
Function Evaluation
Medium
Math
Intermediate Algebra
Question
If f(x) = 2x^2 - 2, what is f(3)?
Answer choices
- 12
- 15
- 4
- 16
- 17
D Correct answer: D) 16
Set up the problem carefully before computing. Whenever you see f(number), substitute that number for x everywhere in the function expression.
Carry the arithmetic through one step at a time, double-checking signs and units. The ACT writes wrong answers to catch the most common slips, so an answer that "looks right" without verification is risky.
Plug the answer back into the original expression as a sanity check whenever the problem allows. If the substitution does not balance, you have made an arithmetic mistake earlier in the work.
The underlying rule
Substitute x = 3. f(3) = 2 · 3^2 - 2 = 2 · 9 - 2 = 18 - 2 = 16.
Why each wrong answer is wrong
- A) 12: A common arithmetic or sign error leads to this value; carefully redo the calculation.
- B) 15: A common arithmetic or sign error leads to this value; carefully redo the calculation.
- C) 4: A common arithmetic or sign error leads to this value; carefully redo the calculation.
- E) 17: A common arithmetic or sign error leads to this value; carefully redo the calculation.
Study tip
f(x) is just a name for an expression; f(3) means substitute 3 for x. The notation looks fancy but the operation is plain substitution.
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