Second derivative test operates via first derivative test

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The conclusions for the conclusive cases of the second derivative test follow from the assumptions via the use of the first derivative test.

In particular, this shows that the first derivative test is stronger than the second derivative test. In other words, in any situation where the second derivative test is applicable and conclusive, so is the first derivative test.

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The proof essentially relies on looking at the proof of the one-sided version of second derivative test, and noting that as part of the proof, we first establish that the conditions hold to apply the one-sided version of the first derivative test.