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Let [tex]$f(x)=\frac{1}{x-3}$[/tex] and [tex]$g(x)=\sqrt{x+5}$[/tex]. What is the domain of [tex][tex]$(f \circ g)(x)$[/tex][/tex]?

[tex]\[
\begin{array}{l}
{[-5,4) \cup(4, \infty)} \\
{[-5,3) \cup(3, \infty)} \\
{(-5,-4) \cup(-4, \infty)} \\
{[-5,-3) \cup(-3, \infty)}
\end{array}
\][/tex]


Sagot :

To determine the domain of [tex]\((f \circ g)(x)\)[/tex], we need to understand the domains of the individual functions [tex]\(f(x)\)[/tex] and [tex]\(g(x)\)[/tex], and then find the restrictions they impose when combined.

1. Domain of [tex]\(g(x) = \sqrt{x+5}\)[/tex]:
- The function [tex]\(g(x)\)[/tex] involves a square root, which is defined only for non-negative values inside the root.
- Therefore, we require:
[tex]\[ x + 5 \geq 0 \][/tex]
- Solving this inequality:
[tex]\[ x \geq -5 \][/tex]
- Thus, the domain of [tex]\(g(x)\)[/tex] is [tex]\([-5, \infty)\)[/tex].

2. Domain of [tex]\(f(x) = \frac{1}{x-3}\)[/tex]:
- The function [tex]\(f(x)\)[/tex] is defined for all [tex]\(x\)[/tex] except where the denominator is zero.
- Therefore, we require:
[tex]\[ x - 3 \neq 0 \quad \Rightarrow \quad x \neq 3 \][/tex]
- So, the domain of [tex]\(f(x)\)[/tex] is [tex]\(\mathbb{R} \setminus \{ 3 \}\)[/tex].

3. Domain of [tex]\((f \circ g)(x)\)[/tex]:
- The composition [tex]\( (f \circ g)(x) \)[/tex] implies that [tex]\( g(x) \)[/tex] must produce values that are within the domain of [tex]\( f \)[/tex].
- Thus, we need:
[tex]\[ g(x) \neq 3 \][/tex]
- We find when [tex]\(g(x) = 3\)[/tex]:
[tex]\[ \sqrt{x + 5} = 3 \][/tex]
- Squaring both sides gives:
[tex]\[ x + 5 = 9 \][/tex]
- Solving for [tex]\(x\)[/tex]:
[tex]\[ x = 4 \][/tex]
- Therefore, [tex]\(x \neq 4\)[/tex] to avoid [tex]\(g(x) = 3\)[/tex].

Combining these restrictions:
- [tex]\(x \geq -5\)[/tex] (from the domain of [tex]\(g(x)\)[/tex])
- [tex]\(x \neq 4\)[/tex] (to ensure [tex]\(g(x) \neq 3\)[/tex])

The domain of [tex]\((f \circ g)(x)\)[/tex] is:
[tex]\[ [-5, 4) \cup (4, \infty) \][/tex]

So, the correct answer is:
[tex]\[ \boxed{[-5, 4) \cup (4, \infty)} \][/tex]
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