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Locate and classify the local extremum using the Second Derivative Test

Consider the function f(x)=xexf(x) = x e^x, defined for all real numbers.

Find the critical number(s) of f(x)=xexf(x) = xe^x, 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 xx-coordinate and the corresponding yy-value).

Hint

Compute f′(x)=ex(x+1)f'(x) = e^x(x+1) using the product rule and set it equal to zero to find the critical number. Then compute f′′(x)f''(x) and evaluate it at that critical number to determine concavity there.

Step-by-step solution

Using the product rule, f′(x)=ex⋅x+ex⋅1=ex(x+1)f'(x) = e^x \cdot x + e^x \cdot 1 = e^x(x+1). Since exe^x is never zero, setting f′(x)=0f'(x)=0 gives x+1=0x+1=0, so the only critical number is x=−1x=-1.

Differentiating again, f′′(x)=ex(x+1)+ex(1)=ex(x+2)f''(x) = e^x(x+1) + e^x(1) = e^x(x+2). Evaluating at x=−1x=-1: f′′(−1)=e−1(1)=e−1>0f''(-1) = e^{-1}(1) = e^{-1} > 0, so by the Second Derivative Test, x=−1x=-1 gives a local minimum. The value is f(−1)=−1⋅e−1=−1/ef(-1) = -1\cdot e^{-1} = -1/e.

Locate the inflection point and confirm concavity change

Find the xx-coordinate of the inflection point of f(x)=xexf(x) = xe^x, and verify that the concavity actually changes there (state the concavity on each side).

Hint

Use f′′(x)=ex(x+2)f''(x) = e^x(x+2) from the previous part, set it equal to zero, and check the sign of f′′f'' on either side of that value to confirm a genuine change in concavity.

Step-by-step solution

From f′′(x)=ex(x+2)f''(x) = e^x(x+2), setting f′′(x)=0f''(x) = 0 gives x=−2x = -2 (since ex≠0e^x \neq 0 for all xx). This is a candidate inflection point that must be checked for an actual sign change.

For x<−2x < -2, say x=−3x=-3: f′′(−3)=e−3(−1)<0f''(-3) = e^{-3}(-1) < 0, so ff is concave down. For x>−2x > -2, say x=0x=0: f′′(0)=e0(2)=2>0f''(0) = e^0(2) = 2 > 0, so ff is concave up. Since concavity changes from down to up at x=−2x=-2, this confirms an inflection point there, located at (−2,−2e−2)(-2, -2e^{-2}).

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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