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Consider these functions:
[tex]$
\begin{array}{l}
f(x)=\frac{1}{3} x^2+4 \\
g(x)=9 x-12
\end{array}
$[/tex]

What is the value of [tex][tex]$g(f(x))$[/tex][/tex]?

A. [tex][tex]$9 x^2-24 x+20$[/tex][/tex]
B. [tex][tex]$3 x$[/tex][/tex]
C. [tex][tex]$3 x^2+24$[/tex][/tex]
D. [tex][tex]$9 x^2+20$[/tex][/tex]

Sagot :

To find the value of [tex]\( g(f(x)) \)[/tex], we need to substitute the function [tex]\( f(x) \)[/tex] into the function [tex]\( g(x) \)[/tex]. Let's begin by detailing each function and how we can compose them.

Given:
[tex]\[ f(x) = \frac{1}{3} x^2 + 4 \][/tex]
[tex]\[ g(x) = 9x - 12 \][/tex]

Step-by-step procedure:

1. Determine [tex]\( f(x) \)[/tex]:
[tex]\[ f(x) = \frac{1}{3} x^2 + 4 \][/tex]

2. Substitute [tex]\( f(x) \)[/tex] into [tex]\( g(x) \)[/tex]:
[tex]\[ g(f(x)) = g\left(\frac{1}{3} x^2 + 4\right) \][/tex]

3. Evaluate [tex]\( g(x) \)[/tex] at [tex]\( \left(\frac{1}{3} x^2 + 4\right) \)[/tex]:
[tex]\[ g\left(\frac{1}{3} x^2 + 4\right) = 9\left(\frac{1}{3} x^2 + 4\right) - 12 \][/tex]

4. Distribute the 9 inside the parentheses:
[tex]\[ 9\left(\frac{1}{3} x^2 + 4\right) = 9 \cdot \frac{1}{3} x^2 + 9 \cdot 4 \][/tex]

5. Simplify the expression:
[tex]\[ 9 \cdot \frac{1}{3} x^2 = 3x^2 \][/tex]
[tex]\[ 9 \cdot 4 = 36 \][/tex]

6. Combine these results:
[tex]\[ 3x^2 + 36 - 12 \][/tex]

7. Simplify further by combining constants:
[tex]\[ 3x^2 + 36 - 12 = 3x^2 + 24 \][/tex]

Thus, the value of [tex]\( g(f(x)) \)[/tex] is:
[tex]\[ \boxed{3 x^2 + 24} \][/tex]

Therefore, the correct answer is:
C. [tex]\( 3 x^2 + 24 \)[/tex]