Example

Example: Computing (fg)(x)(f \cdot g)(x) for f(x)=4x5f(x) = 4x - 5 and g(x)=2x+3g(x) = 2x + 3

Given f(x)=4x5f(x) = 4x - 5 and g(x)=2x+3g(x) = 2x + 3, this example shows how to find the product function (fg)(x)(f \cdot g)(x) and explains the distinction between function multiplication and function composition.

The notation (fg)(x)(f \cdot g)(x) is different from (fg)(x)(f \circ g)(x). In composition, the output of one function is used as the input of another, whereas in multiplication, the two function values are multiplied together.

Using the definition (fg)(x)=f(x)g(x)(f \cdot g)(x) = f(x) \cdot g(x), substitute f(x)=4x5f(x) = 4x - 5 and g(x)=2x+3g(x) = 2x + 3 to get (fg)(x)=(4x5)(2x+3)(f \cdot g)(x) = (4x - 5)(2x + 3). Multiply the binomials to obtain (fg)(x)=8x2+2x15(f \cdot g)(x) = 8x^2 + 2x - 15.

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Updated 2026-05-26

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