ACT · Intermediate Algebra · Function notation and composition
Question act-q-00354
Medium Multiple choice No calculator
The question
If f(x) = 5x² + 2, what is the value of f(5)?
- 127
- 27
- -127
- 128
Show answer & worked solution
Answer: 127
Worked solution. Substitute x = 5: f(5) = 5 × (5)² + 2 = 5 × 25 + 2 = 125 + 2 = 127.
Why each wrong choice is wrong. Forgetting to square the input is the dominant error — the linear-substitution distractor catches it.
Test-day tactic. Substitute with parentheses around the input. (5)² always equals 25, regardless of the sign of 5.
About this question type
Items test substitution into function notation, composition, and (occasionally) inverse functions.
You will see a question shaped like this one on roughly every other official ACT form, typically at a moderate position in the section — solidly within the range that separates a 600-band student from a 700-band student. Treat any miss in this subtopic as a signal to drill the subtopic page before you do another full practice test.