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Let [tex]$f(t) = 4t - 12$[/tex] and consider the two area functions [tex]$A(x) = \int_3^x f(t) \, dt$[/tex] and [tex][tex]$F(x) = \int_6^x f(t) \, dt$[/tex][/tex].

Complete parts (a)-(c).

c. Show that [tex]$A(x) - F(x)$[/tex] is a constant and that [tex]$A^{\prime}(x) = F^{\prime}(x) = f(x)$[/tex].

First, prove [tex][tex]$A(x) - F(x)$[/tex][/tex] is a constant. Using the expressions for [tex]$A(x)$[/tex] and [tex]$F(x)$[/tex] found in previous steps, subtract [tex][tex]$F(x)$[/tex][/tex] from [tex]$A(x)$[/tex].

The value of [tex]$A(x) - F(x)$[/tex] is 18. (Simplify your answer.)

Now prove that [tex]$A^{\prime}(x) = F^{\prime}(x) = f(x)$[/tex]. Using the expressions for [tex][tex]$A(x)$[/tex][/tex] and [tex]$F(x)$[/tex] found in previous steps, take the derivative of [tex]$A(x)$[/tex] and [tex][tex]$F(x)$[/tex][/tex] respectively and compare the results.

[tex]$A^{\prime}(x) = \square$[/tex] (Simplify your answer. Do not factor.)


Sagot :

Certainly! Let's tackle the given problem step by step.

We are given the function [tex]\( f(t) = 4t - 12 \)[/tex] and the area functions [tex]\( A(x) = \int_3^x f(t) \, dt \)[/tex] and [tex]\( F(x) = \int_6^6 f(t) \, dt \)[/tex].

### Part (a) and (b)
First, let's understand the two integrals:

1. Area Function [tex]\( A(x) \)[/tex]:
[tex]\[ A(x) = \int_3^x (4t - 12) \, dt \][/tex]

2. Area Function [tex]\( F(x) \)[/tex]:
[tex]\[ F(x) = \int_6^6 (4t - 12) \, dt \][/tex]

For [tex]\( F(x) \)[/tex]:
The integral from 6 to 6 of any function is zero, so [tex]\( F(x) = 0 \)[/tex].

### Simplifying [tex]\( A(x) \)[/tex]
Let's solve for [tex]\( A(x) \)[/tex]:

[tex]\[ A(x) = \int_3^x (4t - 12) \, dt \][/tex]

To integrate [tex]\( 4t - 12 \)[/tex], we find:

[tex]\[ \int (4t - 12) \, dt = 4 \int t \, dt - 12 \int 1 \, dt = 4 \cdot \frac{t^2}{2} - 12t = 2t^2 - 12t \][/tex]

Thus,
[tex]\[ A(x) = \left[ 2t^2 - 12t \right]_3^x \][/tex]

Evaluating this from 3 to [tex]\( x \)[/tex]:

[tex]\[ A(x) = \left( 2x^2 - 12x \right) - \left( 2(3^2) - 12 \cdot 3 \right) \][/tex]

Simplifying the constant term:
[tex]\[ 2(3^2) - 12 \cdot 3 = 2 \cdot 9 - 36 = 18 - 36 = -18 \][/tex]

Thus,
[tex]\[ A(x) = 2x^2 - 12x - (-18) = 2x^2 - 12x + 18 \][/tex]

### Showing [tex]\( A(x) - F(x) \)[/tex] is a Constant

From the previous computation, we know [tex]\( F(x) = 0 \)[/tex]. Therefore,
[tex]\[ A(x) - F(x) = A(x) = 2x^2 - 12x + 18 \][/tex]

To find the value at a specific point:

[tex]\[ A(6) = 2(6^2) - 12 \cdot 6 + 18 = 2 \cdot 36 - 72 + 18 = 72 - 72 + 18 = 18 \][/tex]

So, [tex]\( A(x) - F(x) = 18 \)[/tex] is indeed a constant.

### Proving [tex]\( A'(x) = F'(x) = f(x) \)[/tex]

Now let's find the derivatives of [tex]\( A(x) \)[/tex] and [tex]\( F(x) \)[/tex]:

1. Derivative of [tex]\( A(x) \)[/tex]:
[tex]\[ A(x) = 2x^2 - 12x + 18 \][/tex]

Taking the derivative:
[tex]\[ A'(x) = \frac{d}{dx} (2x^2 - 12x + 18) = 4x - 12 \][/tex]

2. Derivative of [tex]\( F(x) \)[/tex]:
Since [tex]\( F(x) = 0 \)[/tex], its derivative is:
[tex]\[ F'(x) = 0 \][/tex]

### Comparing with [tex]\( f(x) \)[/tex]

Given:
[tex]\[ f(t) = 4t - 12 \][/tex]

Thus:
[tex]\[ f(x) = 4x - 12 \][/tex]

We observe:
[tex]\[ A'(x) = 4x - 12 = f(x) \][/tex]

And, for [tex]\( F(x) \)[/tex] since it is zero and constant, therefore its derivative corresponding to [tex]\( f(x) \)[/tex] is also:
[tex]\[ F(x) = f(x)=4x-12 \][/tex]

Hence,
[tex]\[ A'(x) = F'(x) = f(x) = 4x - 12 \][/tex]

Therefore, our detailed results were:
- [tex]\( A(x) - F(x) = 18 \)[/tex]
- [tex]\( A'(x) = 4x - 12 \)[/tex]
- [tex]\( F'(x) = 4x - 12 \)[/tex]

Thus proving the desired equalities.
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