Lesson

When we have an equation in two variables then there is a **relation** between the two variables. For example, y=x+1 describes a relation between x and y (that is, y is 1 more than x). Consequently there is also an **inverse relation** between y and x, \, y=x-1 (so y is one less than x).

If, for any value of x there is only one value for y then the relation is a **function**. y=x+1 describes a function, but x^2 + y^2 =1 does not. We can see this because if x=0 then y could be either 2 or -2.

Some, but not all, functions have **inverse functions**. This means that the inverse relation is also a function. Since y=x-1 describes a function, it is the inverse function of y=x+1. However, for example, y=x^2 describes a function, but its inverse relation y^2=x is not a function.

When we say that y is a function of x, we call x the **independent variable** and y the **dependent variable**. We sometimes write functions using function notation. For example, f(x) means f is a function of x. We can then write the inverse function as f^{-1}(x).

To find an inverse function swap the dependent and independent variables. We can then rearrange the equation to make the new dependent variable the subject if necessary.

Find the inverse function of y=-8x+6.

Worked Solution

Idea summary

A **function**, f(x), is a relation where for any value of the independent variable there is only one value of the dependent variable.

An **inverse function**, f^{-1}(x), is an inverse relation which is also a function.

We can find inverse functions and relations by swapping the position of the dependent and independent variables in the equation and then rearranging the equation to make the new dependent variable the subject.

Graphs represent relations in the same way that equations do. Each point on the graph has an x-value and a y-value which satisfies the rules of the relation.

If a graph represents a function then each x-value has only one corresponding y-value. This means that any vertical line will intersect the graph at at most one point. This gives us the **vertical line test**. If we can draw a vertical line which intersects the graph more than once then the graph does not represent a function.

When a function has an inverse each value of the dependent variable has only one possible value for the independent variable. On a graph that means that each y-value has only one corresponding x-value and so any horizontal line will intersect the graph at at most one point. This gives us the **horizontal line test**. If we can draw a horizontal line which intersects the graph more than once then the function does not have an inverse function.

When we want to find the equation of an inverse function, we swap the positions of the independent and dependent variables. Equivalently, to find the graph of an inverse function, we swap the x-coordinates with y-coordinates. This is the same as reflecting the graph about the line y=x.

Do the following graphs have inverse functions?

a

Worked Solution

b

Worked Solution

The lines y=5x (labelled B) and y=x (labelled A) have been plotted below.

By reflecting y=5x about the line y=x, plot the graph of the inverse of y=5x.

Worked Solution

Idea summary

The **vertical line test** says that any vertical line will intersect a function at at most one point.

The **horizontal line test** says that if a function has an inverse function any horizontal line will intersect the function at at most one point.

The graph of an inverse function will be the graph of the original function reflected about the line y=x.