Second Derivative Test - Proof of The Second Derivative Test

Proof of The Second Derivative Test

Suppose we have (the proof for is analogous). Then

Thus, for h sufficiently small we get

which means that if h < 0 so that f is decreasing to the left of x, and that if h > 0 so that f is increasing to the right of x.

Now, by the first derivative test we know that has a local minimum at .

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