Question

For which of the following intervals, the given function f(x) = -2x³ -  9x² - 12x + 1 is decreasing?

(A) (-2, ∞)

(B) (-2, - 1)

(C) (−∞, 1)

(D) (−∞, 2) and (-1, ∞)

Answer 1

Answer is (D) (−∞, 2)  and (-1, ∞)

We have

To be decreasing, we need to have f '(x) < 0, that is,