ACT · Intermediate Algebra · Function notation and composition
Question act-q-00361
Medium Multiple choice No calculator
The question
If f(x) = 5x² − 4, what is the value of f(-1)?
- 1
- -9
- 2
- -1
Show answer & worked solution
Answer: 1
Worked solution. Substitute x = -1: f(-1) = 5 × (-1)² − 4 = 5 × 1 − 4 = 5 − 4 = 1.
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. (-1)² always equals 1, regardless of the sign of -1.
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.