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The function f(x)=−x3+12x2−36x+21f(x) = -x^3 + 12x^2 - 36x + 21 is

A. Increasing on (−∞,2)(-\infty, 2)

B. Decreasing on (−∞,2)(-\infty, 2)

C. Increasing on (2,6)(2, 6)

D. Decreasing on (6,∞)(6, \infty)

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 4

f′(x)=−3x2+24x−36=−3(x−2)(x−6)f'(x)=-3x^2+24x-36=-3(x-2)(x-6). For x<2x<2 and for x>6x>6 the product (x−2)(x−6)(x-2)(x-6) is positive, so f′<0f'<0 and ff decreases. For 2<x<62<x<6 the product is negative, so f′>0f'>0 and ff increases. Hence B, C and D hold.

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