I keep getting “Second-derivative test” questions wrong in Application of Derivatives. What's the trap?
confusing which sign of f'' gives a maximum.
The usual trap: confusing which sign of f'' gives a maximum. How to avoid it: f'' positive = valley (min); f'' negative = hill (max). Go back to the definition — at a critical point, f''>0 means a local minimum (cup up) and f''<0 means a local maximum (cap down).
Key point
Second-derivative test
Common mistake
confusing which sign of f'' gives a maximum.
Memory tip
f'' positive = valley (min); f'' negative = hill (max).
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