Let us consider a relation f: A -> B, where, set A is domain and set B is range.

This relation f is referred as a function, if for each value in domain, there exists exactly one value in the range.

**Horizontal line test indicates whether a function is one-one and inverse of a function is itself a function. **Horizontal line test says that, i

f it is **not** possible to draw a horizontal line which intersects the graph of a function at two or more points, then the graph is one-to-one.

In the above figure, graph passes the horizontal test. Hence, inverse of this function is itself a function.

If it is possible to draw a horizontal line which intersects
graph of a function at two or more points, then the graph is not the graph of a function and inverse of that
function is **not** a function.

In the above figure, graph does not passes the horizontal test. Hence, the inverse of this function is not a function.