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Given: [tex]\( f(x) = 9 \cdot 5^{g(x)} \)[/tex]

where [tex]\( g(x) \)[/tex] is a twice-differentiable function, with the following conditions:
- [tex]\( g(3) = -2 \)[/tex]
- [tex]\( g'(3) = -1 \)[/tex]
- [tex]\( g''(3) = -9 \)[/tex]

Find the value of [tex]\( f'(3) \)[/tex].

A. [tex]\(-1.218\)[/tex]
B. [tex]\(-0.5794\)[/tex]
C. None of the other options are correct
D. [tex]\(-0.0644\)[/tex]
E. [tex]\(-0.2516\)[/tex]
F. [tex]\(-0.36\)[/tex]


Sagot :

To find the derivative [tex]\( f'(x) \)[/tex] of the function [tex]\( f(x) = 9 \cdot 5^{g(x)} \)[/tex] at [tex]\( x = 3 \)[/tex], we will follow these steps:

1. Express [tex]\( f(x) \)[/tex] and the given information:
[tex]\[ f(x) = 9 \cdot 5^{g(x)} \][/tex]
Given data:
[tex]\[ g(3) = -2, \quad g'(3) = -1, \quad g''(3) = -9 \][/tex]

2. Differentiate [tex]\( f(x) \)[/tex] using the chain rule:
To find [tex]\( f'(x) \)[/tex], we apply the chain rule. First, express [tex]\( f(x) \)[/tex] as:
[tex]\[ f(x) = 9 \cdot 5^{g(x)} \][/tex]
Taking the derivative with respect to [tex]\( x \)[/tex], we get:
[tex]\[ f'(x) = 9 \cdot \frac{d}{dx} \left( 5^{g(x)} \right) \][/tex]

3. Differentiate [tex]\( 5^{g(x)} \)[/tex] using the chain rule and the exponential function derivative:
Recall that the derivative of [tex]\( a^{h(x)} \)[/tex] with respect to [tex]\( x \)[/tex] is [tex]\( a^{h(x)} \ln(a) h'(x) \)[/tex]. Applying this, we find:
[tex]\[ \frac{d}{dx} \left( 5^{g(x)} \right) = 5^{g(x)} \cdot \ln(5) \cdot g'(x) \][/tex]
Hence:
[tex]\[ f'(x) = 9 \cdot 5^{g(x)} \cdot \ln(5) \cdot g'(x) \][/tex]

4. Evaluate [tex]\( f'(x) \)[/tex] at [tex]\( x = 3 \)[/tex]:
Plug in the given values [tex]\( g(3) = -2 \)[/tex] and [tex]\( g'(3) = -1 \)[/tex]:
[tex]\[ f'(3) = 9 \cdot 5^{g(3)} \cdot \ln(5) \cdot g'(3) \][/tex]
Substitute the known values:
[tex]\[ f'(3) = 9 \cdot 5^{-2} \cdot \ln(5) \cdot (-1) \][/tex]

5. Simplify the expression:
[tex]\[ 5^{-2} = \frac{1}{5^2} = \frac{1}{25} \][/tex]
Therefore:
[tex]\[ f'(3) = 9 \cdot \frac{1}{25} \cdot \ln(5) \cdot (-1) \][/tex]
[tex]\[ f'(3) = -\frac{9}{25} \cdot \ln(5) \][/tex]

6. Approximate the numerical value:
Given the options, we see that the closest numerical value to our simplified expression of [tex]\( f'(3) \)[/tex] is approximately:
[tex]\[ f'(3) \approx -0.5794 \][/tex]

So, the value of [tex]\( f'(3) \)[/tex] is closest to:
[tex]\[ \boxed{-0.5794} \][/tex]
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