Let [tex]$f(x) = x^3 + 3x^2 - 45x + 11$[/tex].

Sketch the function using a graphing tool and answer the following questions:

(a) Use the definition of a derivative or the derivative rules to find [tex]f'(x) = \square[/tex]

(b) Use the definition of a derivative or the derivative rules to find [tex]f''(x) = \square[/tex]

(c) On what interval is [tex]f[/tex] increasing (include the endpoints in the interval)?
[tex]\[ \text{Interval of increasing} = \square \][/tex]

(d) On what interval is [tex]f[/tex] decreasing (include the endpoints in the interval)?
[tex]\[ \text{Interval of decreasing} = \square \][/tex]

(e) On what interval is [tex]f[/tex] concave downward (include the endpoints in the interval)?
[tex]\[ \text{Interval of downward concavity} = \square \][/tex]

(f) On what interval is [tex]f[/tex] concave upward (include the endpoints in the interval)?
[tex]\[ \text{Interval of upward concavity} = \square \][/tex]



Answer :

To solve the problem step-by-step, we'll follow these steps:
1. Find the first derivative of [tex]\( f(x) \)[/tex].
2. Find the second derivative of [tex]\( f(x) \)[/tex].
3. Determine the intervals where [tex]\( f(x) \)[/tex] is increasing or decreasing.
4. Determine the intervals where [tex]\( f(x) \)[/tex] is concave upward or downward.

Given:
[tex]\[ f(x) = x^3 + 3x^2 - 45x + 11 \][/tex]

### (a) Find the first derivative of [tex]\( f(x) \)[/tex]

Using the power rule [tex]\(\left(\frac{d}{dx} x^n = nx^{n-1}\right)\)[/tex]:

[tex]\[ f'(x) = \frac{d}{dx}\left(x^3 + 3x^2 - 45x + 11\right) = 3x^2 + 6x - 45 \][/tex]

So,
[tex]\[ f'(x) = 3x^2 + 6x - 45 \][/tex]

### (b) Find the second derivative of [tex]\( f(x) \)[/tex]

Again using the power rule:

[tex]\[ f''(x) = \frac{d}{dx}\left(3x^2 + 6x - 45\right) = 6x + 6 \][/tex]

So,
[tex]\[ f''(x) = 6x + 6 \][/tex]

### (c) Determine the intervals where [tex]\( f(x) \)[/tex] is increasing

First, find the critical points by setting [tex]\( f'(x) = 0 \)[/tex]:

[tex]\[ 3x^2 + 6x - 45 = 0 \][/tex]

Divide the equation by 3:

[tex]\[ x^2 + 2x - 15 = 0 \][/tex]

Factor:

[tex]\[ (x + 5)(x - 3) = 0 \][/tex]

So, the critical points are:

[tex]\[ x = -5 \quad \text{and} \quad x = 3 \][/tex]

To determine the intervals of increase and decrease, test the sign of [tex]\( f'(x) \)[/tex] in the intervals determined by the critical points [tex]\((- \infty, -5)\)[/tex], [tex]\((-5, 3)\)[/tex], and [tex]\((3, \infty)\)[/tex]:

- For [tex]\( x \in (-\infty, -5) \)[/tex], choose [tex]\( x = -6 \)[/tex]:
[tex]\[ f'(-6) = 3(-6)^2 + 6(-6) - 45 = 108 - 36 - 45 = 27 \quad (\text{positive}) \][/tex]

- For [tex]\( x \in (-5, 3) \)[/tex], choose [tex]\( x = 0 \)[/tex]:
[tex]\[ f'(0) = 3(0)^2 + 6(0) - 45 = -45 \quad (\text{negative}) \][/tex]

- For [tex]\( x \in (3, \infty) \)[/tex], choose [tex]\( x = 4 \)[/tex]:
[tex]\[ f'(4) = 3(4)^2 + 6(4) - 45 = 48 + 24 - 45 = 27 \quad (\text{positive}) \][/tex]

So, [tex]\( f(x) \)[/tex] is increasing on:

[tex]\[ (-\infty, -5] \cup [3, \infty) \][/tex]

### (d) Determine the intervals where [tex]\( f(x) \)[/tex] is decreasing

Using the results from (c), [tex]\( f(x) \)[/tex] is decreasing on:

[tex]\[ [-5, 3] \][/tex]

### (e) Determine the intervals where [tex]\( f(x) \)[/tex] is concave downward

Find the inflection points by setting [tex]\( f''(x) = 0 \)[/tex]:

[tex]\[ 6x + 6 = 0 \][/tex]
[tex]\[ x = -1 \][/tex]

Test the sign of [tex]\( f''(x) \)[/tex] in the intervals [tex]\((- \infty, -1)\)[/tex] and [tex]\((-1, \infty)\)[/tex]:

- For [tex]\( x \in (-\infty, -1) \)[/tex], choose [tex]\( x = -2 \)[/tex]:
[tex]\[ f''(-2) = 6(-2) + 6 = -12 + 6 = -6 \quad (\text{negative}) \][/tex]

- For [tex]\( x \in (-1, \infty) \)[/tex], choose [tex]\( x = 0 \)[/tex]:
[tex]\[ f''(0) = 6(0) + 6 = 6 \quad (\text{positive}) \][/tex]

So, [tex]\( f(x) \)[/tex] is concave downward on:

[tex]\[ (-\infty, -1] \][/tex]

### (f) Determine the intervals where [tex]\( f(x) \)[/tex] is concave upward

Using the results from (e), [tex]\( f(x) \)[/tex] is concave upward on:

[tex]\[ [-1, \infty) \][/tex]

### Summary:

- [tex]\((a)\)[/tex] [tex]\( f'(x) = 3x^2 + 6x - 45 \)[/tex]
- [tex]\((b)\)[/tex] [tex]\( f''(x) = 6x + 6 \)[/tex]
- [tex]\((c)\)[/tex] Interval of increasing: [tex]\( (-\infty, -5] \cup [3, \infty) \)[/tex]
- [tex]\((d)\)[/tex] Interval of decreasing: [tex]\( [-5, 3] \)[/tex]
- [tex]\((e)\)[/tex] Interval of concave downward: [tex]\( (-\infty, -1] \)[/tex]
- [tex]\((f)\)[/tex] Interval of concave upward: [tex]\( [-1, \infty) \)[/tex]