### (A) List all critical numbers of [tex]\( f \)[/tex].
If there are no critical numbers, enter 'NONE'.

Critical numbers:

### (B) Use interval notation to indicate where [tex]\( f(x) \)[/tex] is increasing.
Note: Use 'Inf' for [tex]\( \infty \)[/tex], '-Inf' for [tex]\( -\infty \)[/tex], and use 'U' for the union symbol.

Increasing:

### (C) Use interval notation to indicate where [tex]\( f(x) \)[/tex] is decreasing.

Decreasing:

### (D) List the [tex]\( x \)[/tex]-coordinates of all local maxima of [tex]\( f \)[/tex].
If there are no local maxima, enter 'NONE'.

[tex]\( x \)[/tex] values of local maxima:

### (E) List the [tex]\( x \)[/tex]-coordinates of all local minima of [tex]\( f \)[/tex].
If there are no local minima, enter 'NONE'.

[tex]\( x \)[/tex] values of local minima:



Answer :

Sure, let's solve the problem step-by-step.

Given the function:
[tex]$ f(x) = \frac{7}{x^2 - 49} $[/tex]

### (A) Find all critical numbers of [tex]\( f \)[/tex]

To find the critical numbers, we first need to find the derivative of [tex]\( f(x) \)[/tex], which is [tex]\( f'(x) \)[/tex], and then set it equal to zero or find points where it is undefined.

First, rewrite [tex]\( f(x) \)[/tex]:
[tex]$ f(x) = \frac{7}{(x-7)(x+7)} $[/tex]

Let's find the derivative [tex]\( f'(x) \)[/tex]. Using the quotient rule:
If [tex]\( f(x) = \frac{u(x)}{v(x)} \)[/tex], then [tex]\( f'(x) = \frac{u'v - uv'}{v^2} \)[/tex].

Here, [tex]\( u(x) = 7 \)[/tex] and [tex]\( v(x) = x^2 - 49 \)[/tex], thus:
[tex]\[ u' = 0 \][/tex]
[tex]\[ v' = 2x \][/tex]

Applying the quotient rule:
[tex]\[ f'(x) = \frac{0 \cdot (x^2 - 49) - 7 \cdot 2x}{(x^2 - 49)^2} \][/tex]
[tex]\[ f'(x) = \frac{-14x}{(x^2 - 49)^2} \][/tex]

Set [tex]\( f'(x) = 0 \)[/tex]:
[tex]\[ \frac{-14x}{(x^2 - 49)^2} = 0 \][/tex]
This happens when [tex]\( -14x = 0 \)[/tex], which implies:
[tex]\[ x = 0 \][/tex]

We also need to check where [tex]\( f'(x) \)[/tex] is undefined. Since [tex]\( f'(x) = \frac{-14x}{(x^2 - 49)^2} \)[/tex], it will be undefined where the denominator is zero.
[tex]\[ x^2 - 49 = 0 \][/tex]
[tex]\[ x = \pm 7 \][/tex]

So, the function [tex]\( f(x) \)[/tex] is undefined at [tex]\( x = 7 \)[/tex] and [tex]\( x = -7 \)[/tex].

Hence, the only critical number in the domain of [tex]\( f(x) \)[/tex] is [tex]\( x = 0 \)[/tex].

Critical numbers = [tex]\( 0 \)[/tex]

### (B) Use interval notation to indicate where [tex]\( f(x) \)[/tex] is increasing

We use the first derivative [tex]\( f'(x) = \frac{-14x}{(x^2 - 49)^2} \)[/tex] to determine where the function is increasing or decreasing.

Check the sign of [tex]\( f'(x) \)[/tex] on the intervals created by the critical points and points of discontinuity.

1. Interval [tex]\((- \infty, -7)\)[/tex]:
Pick a test point, say [tex]\( x = -8 \)[/tex]:
[tex]\[ f'(-8) = \frac{-14(-8)}{((-8)^2 - 49)^2} > 0 \][/tex]

2. Interval [tex]\((-7, 0)\)[/tex]:
Pick a test point, say [tex]\( x = -1 \)[/tex]:
[tex]\[ f'(-1) = \frac{-14(-1)}{((-1)^2 - 49)^2} > 0 \][/tex]

3. Interval [tex]\((0, 7)\)[/tex]:
Pick a test point, say [tex]\( x = 1 \)[/tex]:
[tex]\[ f'(1) = \frac{-14(1)}{((1)^2 - 49)^2} < 0 \][/tex]

4. Interval [tex]\((7, \infty)\)[/tex]:
Pick a test point, say [tex]\( x = 8 \)[/tex]:
[tex]\[ f'(8) = \frac{-14(8)}{((8)^2 - 49)^2} < 0 \][/tex]

From the test points:
- [tex]\( f(x) \)[/tex] is increasing on the intervals [tex]\((- \infty, -7) \cup (-7, 0)\)[/tex].

Increasing: [tex]\( (- \infty, -7) U (-7, 0) \)[/tex]

### (C) Use interval notation to indicate where [tex]\( f(x) \)[/tex] is decreasing

From the analysis above:
- [tex]\( f(x) \)[/tex] is decreasing on the intervals [tex]\( (0, 7) \cup (7, \infty) \)[/tex].

Decreasing: [tex]\( (0, 7) U (7, \infty) \)[/tex]

### (D) List the [tex]\( x \)[/tex]-coordinates of all local maxima

A local maximum would occur where [tex]\( f'(x) \)[/tex] changes from positive to negative. From the intervals above and the nature of [tex]\( f'(x) \)[/tex]:

There is no point where [tex]\( f'(x) changes from positive to negative. Local maxima: \( NONE \)[/tex]

### (E) List the [tex]\( x \)[/tex]-coordinates of all local minima

A local minimum would occur where [tex]\( f'(x) \)[/tex] changes from negative to positive.

The only critical point is [tex]\( x = 0 \)[/tex], and we observed:
[tex]\( f'(x) \)[/tex] changes from positive to negative, indicating no local minima at [tex]\( x = 0 \)[/tex].

Local minima: [tex]\( NONE \)[/tex]