Locate and classify the local extremum using the Second Derivative Test
Consider the function , defined for all real numbers.
Find the critical number(s) of , then use the Second Derivative Test to classify each critical point as a local maximum or local minimum. State the local extreme value (both the -coordinate and the corresponding -value).
Hint
Compute using the product rule and set it equal to zero to find the critical number. Then compute and evaluate it at that critical number to determine concavity there.
Step-by-step solution
Using the product rule, . Since is never zero, setting gives , so the only critical number is .
Differentiating again, . Evaluating at : , so by the Second Derivative Test, gives a local minimum. The value is .
Locate the inflection point and confirm concavity change
Find the -coordinate of the inflection point of , and verify that the concavity actually changes there (state the concavity on each side).
Hint
Use from the previous part, set it equal to zero, and check the sign of on either side of that value to confirm a genuine change in concavity.
Step-by-step solution
From , setting gives (since for all ). This is a candidate inflection point that must be checked for an actual sign change.
For , say : , so is concave down. For , say : , so is concave up. Since concavity changes from down to up at , this confirms an inflection point there, located at .
Comparing the locations of the local minimum and the inflection point
Hint
Step-by-step solution
The local minimum occurs at x equals negative one, while the inflection point occurs at x equals negative two. Since negative two is farther from zero in the negative direction, the inflection point lies farther to the left. This directly follows from the critical number found in part a and the inflection point found in part b.
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