Horizontal-line Test : If every horizontal line intersects the graph of a function in at most one point, then is one-to-one function.

Observe the above diagram :

Since every horizontal line intersects the graph of a function in at most one point, the function is one-to-one function.

It would be easy to find whether the function is one-to-one function or not using horizontal line test.

It would be easy to find whether the function is one-to-one function or not using horizontal line test.