Math Problem Statement

For f(x) = x / (x + 1) and g(x) = 9 / x, find (f o g)(x) and the domain of f o g.

Solution

Let's break down the problem based on the image you uploaded.

We are given the following functions:

  • f(x)=xx+1f(x) = \frac{x}{x+1}
  • g(x)=9xg(x) = \frac{9}{x}

We are asked to find:

  1. (fg)(x)(f \circ g)(x), which is f(g(x))f(g(x))
  2. The domain of fgf \circ g

Part (a): Finding (fg)(x)(f \circ g)(x)

To compute f(g(x))f(g(x)), substitute g(x)g(x) into f(x)f(x):

f(g(x))=f(9x)=9x9x+1f(g(x)) = f\left(\frac{9}{x}\right) = \frac{\frac{9}{x}}{\frac{9}{x} + 1}

Now, simplify the expression:

f(g(x))=9x9x+1=9x9+xx=99+xf(g(x)) = \frac{\frac{9}{x}}{\frac{9}{x} + 1} = \frac{\frac{9}{x}}{\frac{9 + x}{x}} = \frac{9}{9 + x}

Thus, (fg)(x)=99+x(f \circ g)(x) = \frac{9}{9 + x}.

Part (b): Finding the Domain of fgf \circ g

To find the domain of fgf \circ g, we need to consider the domains of both functions:

  1. Domain of g(x)g(x): Since g(x)=9xg(x) = \frac{9}{x}, x0x \neq 0, so the domain of g(x)g(x) is (,0)(0,)(-\infty, 0) \cup (0, \infty).

  2. Domain of f(g(x))f(g(x)): For f(g(x))=99+xf(g(x)) = \frac{9}{9 + x}, the expression 9+x09 + x \neq 0, which gives x9x \neq -9.

So, the domain of fgf \circ g is all values of xx where x0x \neq 0 and x9x \neq -9. Therefore, the domain is:

(,9)(9,0)(0,)(-\infty, -9) \cup (-9, 0) \cup (0, \infty)

Final Answers:

  • (fg)(x)=99+x(f \circ g)(x) = \frac{9}{9 + x}
  • Domain: (,9)(9,0)(0,)(-\infty, -9) \cup (-9, 0) \cup (0, \infty)

Would you like further details or have any questions?

Here are 5 follow-up questions to expand on this:

  1. What is the general process of finding the domain of a composition of functions?
  2. How do you approach simplifying complex rational functions like f(g(x))f(g(x))?
  3. What would happen to the domain if the form of g(x)g(x) changed?
  4. Can you apply this same method to other types of function compositions (e.g., logarithmic or trigonometric)?
  5. How does the presence of an asymptote at x=0x = 0 affect the domain?

Tip: Always check the domains of both functions when dealing with compositions to avoid missing important restrictions!

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Math Problem Analysis

Mathematical Concepts

Function Composition
Rational Functions
Domain of Functions

Formulas

(f o g)(x) = f(g(x))
Domain: Exclude values that make the denominator zero in both f(x) and g(x)

Theorems

Domain Restriction Theorem for Rational Functions

Suitable Grade Level

Grades 10-12