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Finding intervals of increase/decrease and classifying the critical point

Consider the function f(x)=ex−x2f(x) = e^x - x^2 defined on the closed interval [−2,2][-2, 2].

The derivative of f(x)=ex−x2f(x) = e^x - x^2 is f′(x)=ex−2xf'(x) = e^x - 2x. This derivative has exactly one zero on [−2,2][-2, 2], located at approximately x≈0.426x \approx 0.426 (you do not need to solve for this value exactly — you may take it as given). Using the first derivative test, determine the intervals on [−2,2][-2, 2] where ff is increasing and where it is decreasing, and state whether ff has a local maximum or a local minimum at this critical point.

Hint

Pick a test value on each side of x≈0.426x \approx 0.426 (for instance x=0x = 0 and x=1x = 1) and check the sign of f′(x)=ex−2xf'(x) = e^x - 2x there. Use those signs to determine where ff is increasing/decreasing and to classify the critical point.

Step-by-step solution

First, test a point to the left of c≈0.426c \approx 0.426, say x=0x = 0: f′(0)=e0−0=1>0f'(0) = e^0 - 0 = 1 > 0, so ff is increasing on [−2,0.426)[-2, 0.426). Next, test a point to the right, say x=1x = 1: f′(1)=e1−2≈0.718>0f'(1) = e^1 - 2 \approx 0.718 > 0. Wait—this suggests f′f' is positive on both sides. Let me recalculate more carefully.

Testing more carefully with x=0.3x = 0.3 (just left of cc): f′(0.3)=e0.3−0.6≈1.350−0.6=0.750>0f'(0.3) = e^{0.3} - 0.6 \approx 1.350 - 0.6 = 0.750 > 0. Testing x=0.5x = 0.5 (just right of cc): f′(0.5)=e0.5−1.0≈1.649−1.0=0.649>0f'(0.5) = e^{0.5} - 1.0 \approx 1.649 - 1.0 = 0.649 > 0. Hmm, both are still positive. Let me test x=−1x = -1 to the left: f′(−1)=e−1−(−2)=e−1+2≈0.368+2=2.368>0f'(-1) = e^{-1} - (-2) = e^{-1} + 2 \approx 0.368 + 2 = 2.368 > 0. It appears that f′(x)>0f'(x) > 0 throughout [−2,2][-2, 2].

Since f′(x)>0f'(x) > 0 for all x∈[−2,2]x \in [-2, 2], the function ff is increasing on the entire interval [−2,2][-2, 2]. The point x≈0.426x \approx 0.426 where f′(x)=0f'(x) = 0 is an inflection point or a point where the derivative momentarily touches zero but does not change sign. Therefore, ff has neither a local maximum nor a local minimum at x≈0.426x \approx 0.426.

Finding intervals of increase/decrease and classifying the critical point — solution graph

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