This property is the core of the inverse function definition: the inverse function "undoes" the effect of the original function, and vice versa. If we compose a function with its inverse (in any order), we get the identity function I(x)=x.
Composition of f with :
f−1
(f∘f−1)(x)=f(f−1(x))=x
This holds for all x in the domain of f−1 (which is the range of f).
Composition of f−1 with f:
(f−1∘f)(x)=f−1(f(x))=x
This holds for all x in the domain of f.
Example:
We know that if f(x)=2x+3, its inverse is f−1(x)=2x−3. Let's verify the composition property:
If we have a composition of two functions, both of which have inverses, the inverse of the composition is the composition of their inverses, but in reverse order.
Let f and g be two functions with inverses f−1 and g−1. Then the inverse of the composition f∘g is:
(f∘g)−1(x)=(g−1∘f−1)(x)
Note the reversed order: g−1 is applied first, then f−1.
Imagine putting on socks (g) and then shoes (f). To undo this (the inverse), you must take off the shoes (f−1) first, then take off the socks (g−1). The order is reversed.
Example:
Let f(x)=x+1 (its inverse is f−1(x)=x−1) and g(x)=3x (its inverse is g−1(x)=3x).
Find (f∘g)(x):
(f∘g)(x)=f(g(x))=f(3x)=3x+1
Find the inverse of (f∘g)(x):
Let y=3x+1. Swap x and y: .
Find (g−1∘f−1)(x):
(g−1∘f−1)(x)=g−1(f−1(x))=g−1(x−1)
Since the results from steps 2 and 3 are the same, it is proven that (f∘g)−1(x)=(g−1∘f−1)(x).
The domain of the original function f becomes the range of its inverse function f−1, and the range of the original function f becomes the domain of its inverse function f−1.