Question

The graph of the function f(x) = –(x + 6)(x + 2) is shown below.Which statement about the function is true?

The function is increasing for all real values of x where
x < –4.
The function is increasing for all real values of x where
–6 < x < –2.
The function is decreasing for all real values of x where
x < –6 and where x > –2.
The function is decreasing for all real values of x where
x < –4.

Answer 1

The first statement is true.

Explanation:

The function is f(x) = - (x + 6)(x + 2)

⇒ f(x) = - x² - 8x - 12

Now, condition for a function f(x) to be increasing at x = a is f'(a) > 0.

Now, f(x) = - x² - 8x - 12

⇒ f'(x) = -2x - 8 {Differentiating with respect to x}

Now, f'(a) = -2a - 8 {Here a can be any real value}

And, the condition for increasing function at x = a is

- 2a - 8 > 0

⇒ - 2a > 8

⇒ a < - 4

Therefore, the first statement is true i.e. the function is increasing for all real values of x where x < – 4. (Answer)