Let us consider a relation f: A -> B

Where, A is domain and B is range. This relation f is a function, if each value in domain A is associated with exactly one value in range B.

Vertical line test determines if a relation can be defined as a function.

If it is **not**
possible to draw a vertical line which intersects graph of a relation at two or more points, then that relation is a
function.In the above figure, graph passes vertical test, hence this relation is a function.

If it is possible to draw a vertical line which intersects
graph of a relation at two or more points, then that
relation is **not** a function.In the above figure, graph fails vertical test, hence this relation is not a function.

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