Advanced MathEasyEvaluating a quadratic at a negative input
Advanced Math practice question
The function f is defined by f(x) = −x² + 4x. What is the value of f(−1)?
- A. −5Correct
- B. 3
- C. 5
- D. −3
Answer: A. −5
f(−1) = −(−1)² + 4(−1) = −(1) + (−4) = −5. Note that (−1)² = 1, and the leading minus sign then makes it −1.
Why the other answers are wrong
- B. 3
- 3 comes from −1 + 4, treating 4x as +4 when x = −1. 4(−1) = −4.
- C. 5
- 5 comes from +1 + 4: both the sign in front of x² and the sign of x were dropped.
- D. −3
- −3 comes from +1 − 4, reading −x² as (−x)² = 1 instead of −(x²) = −1.
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