Math Problem Statement

If f(x) = √(x+1), g(x) = 2/(x-3), then find the domain of (f ∘ g)(x).

Solution

We need to find the domain of the composite function (fg)(x)=f(g(x))(f \circ g)(x) = f(g(x)) given that:

f(x)=x+1andg(x)=2x3f(x) = \sqrt{x+1} \quad \text{and} \quad g(x) = \frac{2}{x - 3}

Step 1: Identify the domain of g(x)g(x)

The function g(x)=2x3g(x) = \frac{2}{x - 3} is undefined when the denominator is zero. Therefore, g(x)g(x) is undefined at x=3x = 3. Thus, the domain of g(x)g(x) is: x(,3)(3,)x \in (-\infty, 3) \cup (3, \infty)

Step 2: Identify the domain of f(x)f(x)

For f(x)=x+1f(x) = \sqrt{x+1}, the expression inside the square root must be non-negative: x+10x1x + 1 \geq 0 \Rightarrow x \geq -1 Therefore, the domain of f(x)f(x) is: x[1,)x \in [-1, \infty)

Step 3: Find the domain of (fg)(x)(f \circ g)(x)

The composite function (fg)(x)=f(g(x))(f \circ g)(x) = f(g(x)) is defined only when:

  1. g(x)g(x) is defined.
  2. g(x)g(x) produces values within the domain of f(x)f(x).

Since the domain of f(x)f(x) is x1x \geq -1, we need: g(x)1g(x) \geq -1 Substitute g(x)=2x3g(x) = \frac{2}{x - 3} into this inequality: 2x31\frac{2}{x - 3} \geq -1 Solving this inequality will give us the valid xx-values that make f(g(x))f(g(x)) defined.

Solving 2x31\frac{2}{x - 3} \geq -1

  1. Multiply both sides by x3x - 3 (considering sign changes):
    • If x3>0x - 3 > 0 (i.e., x>3x > 3), we get 21(x3)2 \geq -1(x - 3).
    • If x3<0x - 3 < 0

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Math Problem Analysis

Mathematical Concepts

Functions
Composite Functions
Domain of a Function
Square Root Function
Rational Function

Formulas

f(g(x)) = f(g(x))
Domain restrictions based on function composition

Theorems

Domain of composite functions

Suitable Grade Level

Grades 10-12