Answer: Choice D
[tex](f \circ g)(x) = x+12[/tex]
Domain = [-3, infinity)
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Work Shown:
[tex]f(x) = x^2 + 9\\\\f(g(x)) = (g(x))^2 + 9\\\\f(g(x)) = (\sqrt{x+3})^2 + 9\\\\f(g(x)) = x+3 + 9\\\\f(g(x)) = x+12\\\\(f \circ g)(x) = x+12\\\\[/tex]
In step 2, we replaced every x with g(x)
In step 3, we plugged in g(x) = sqrt(x+3)
The domain of g(x) is [-3, infinity), so this is the domain of [tex](f \circ g)(x)[/tex] as well since the composite function depends entirely on g(x). Put another way: the input of f(x) depends on the output of g(x), so that's why the domains match up.
