Question

find the intervals on which ( f(x) ) is increasing, the intervals on which ( f(x) ) is decreasing and the local extrema.
( f(x)=-2 x^{2}-8 x - 21 )
select the correct choice below and, if necessary, fill in the answer box to complete your choice.
a. the function is increasing on
(type your answer in interval notation. type integers or simplified fractions. use a comma to separate answers as needed.)
b. the function is never increasing.

Explanation:

Step1: Find the derivative of \(f(x)\)

The derivative of \(f(x)=-2x^{2}-8x - 21\) is \(f^{\prime}(x)=-4x - 8\) (using the power rule \((x^{n})^\prime=nx^{n - 1}\)).

Step2: Find the critical point

Set \(f^{\prime}(x)=0\), so \(-4x-8 = 0\).
Solve for \(x\):

$$
LATEXBLOCK0
$$

Step3: Determine the sign of \(f^{\prime}(x)\) in intervals

• For \(x<-2\) (e.g., \(x=-3\)), \(f^{\prime}(-3)=-4\times(-3)-8=12 - 8=4>0\).
• For \(x>-2\) (e.g., \(x = 0\)), \(f^{\prime}(0)=-4\times0-8=-8<0\).

Answer:

A. The function is increasing on \((-\infty,-2)\)