Math Problem Statement

Let f ( x )

1 x − 5 f ( x )

1 x

5 and g ( x )

2 x + 5 g ( x )

2 x + 5 .

Find the following functions. Simplify your answers.

f ( g ( x ) )

f ( g ( x ) )

,

g ( f ( x ) )

g ( f ( x ) )

.

Solution

We are given two functions:

f(x)=1x5f(x) = \frac{1}{x} - 5 and g(x)=2x+5.g(x) = \frac{2}{x} + 5.

We are asked to find two composite functions: f(g(x))f(g(x)) and g(f(x))g(f(x)).

1. Find f(g(x))f(g(x)):

To find f(g(x))f(g(x)), we substitute g(x)g(x) into the function f(x)f(x):

f(g(x))=f(2x+5).f(g(x)) = f\left(\frac{2}{x} + 5\right).

Now, substitute 2x+5\frac{2}{x} + 5 into f(x)=1x5f(x) = \frac{1}{x} - 5:

f(g(x))=12x+55.f(g(x)) = \frac{1}{\frac{2}{x} + 5} - 5.

Simplify the first term:

12x+5=x2+5x.\frac{1}{\frac{2}{x} + 5} = \frac{x}{2 + 5x}.

Thus,

f(g(x))=x2+5x5.f(g(x)) = \frac{x}{2 + 5x} - 5.

Now subtract 55 from the expression:

f(g(x))=x2+5x5(2+5x)2+5x=x5(2+5x)2+5x.f(g(x)) = \frac{x}{2 + 5x} - \frac{5(2 + 5x)}{2 + 5x} = \frac{x - 5(2 + 5x)}{2 + 5x}.

Simplifying the numerator:

x5(2+5x)=x1025x=24x10.x - 5(2 + 5x) = x - 10 - 25x = -24x - 10.

Thus,

f(g(x))=24x102+5x.f(g(x)) = \frac{-24x - 10}{2 + 5x}.

2. Find g(f(x))g(f(x)):

Now, substitute f(x)=1x5f(x) = \frac{1}{x} - 5 into g(x)=2x+5g(x) = \frac{2}{x} + 5:

g(f(x))=g(1x5).g(f(x)) = g\left(\frac{1}{x} - 5\right).

Substitute into g(x)g(x):

g(f(x))=21x5+5.g(f(x)) = \frac{2}{\frac{1}{x} - 5} + 5.

Simplify the first term:

21x5=2x15x.\frac{2}{\frac{1}{x} - 5} = \frac{2x}{1 - 5x}.

Thus,

g(f(x))=2x15x+5.g(f(x)) = \frac{2x}{1 - 5x} + 5.

Now express 55 as 5(15x)15x\frac{5(1 - 5x)}{1 - 5x}:

g(f(x))=2x15x+5(15x)15x=2x+5(15x)15x.g(f(x)) = \frac{2x}{1 - 5x} + \frac{5(1 - 5x)}{1 - 5x} = \frac{2x + 5(1 - 5x)}{1 - 5x}.

Simplifying the numerator:

2x+5(15x)=2x+525x=23x+5.2x + 5(1 - 5x) = 2x + 5 - 25x = -23x + 5.

Thus,

g(f(x))=23x+515x.g(f(x)) = \frac{-23x + 5}{1 - 5x}.

Final Results:

f(g(x))=24x102+5xf(g(x)) = \frac{-24x - 10}{2 + 5x} and g(f(x))=23x+515x.g(f(x)) = \frac{-23x + 5}{1 - 5x}.

Would you like any further details or clarifications?

Here are some related questions you might consider:

  1. What is the domain of f(g(x))f(g(x))?
  2. What is the domain of g(f(x))g(f(x))?
  3. Can you find the inverse of f(x)f(x)?
  4. Can you find the inverse of g(x)g(x)?
  5. What happens when you compose f(x)f(x) and g(x)g(x) in the opposite order?

Tip: When composing functions, always check the domains to ensure the resulting function is valid!

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Math Problem Analysis

Mathematical Concepts

Function Composition
Algebra
Rational Functions

Formulas

f(g(x)) = f(g(x)) = f(2/x + 5) = 1/(2/x + 5) - 5
g(f(x)) = g(f(x)) = g(1/x - 5) = 2/(1/x - 5) + 5

Theorems

Function Composition Theorem

Suitable Grade Level

Grades 10-12