Horizontal Line Test

The horizontal line test is a convenient method that can determine whether a given function has an inverse, but more importantly to find out if the inverse is also a function.

Remember that it is very possible that a function may have an inverse but at the same time, the inverse is not a function because it doesn’t pass the vertical line test.

So here’s the deal!

If the horizontal line intersects the graph of a function in all places at exactly one point, then the given function should have an inverse that is also a function. We say this function passes the horizontal line test.


Here are some examples of functions that pass the horizontal line test:

Horizontal Line Cutting or Hitting the Graph at Exactly One Point

  • Graph of the line f\left( x \right) = - x + 2.

  • Graph of the square root function f\left( x \right) = \sqrt {x + 1}


  • Graph of the rational function f\left( x \right) = {1 \over {x + 1}}.


On the other hand, if the horizontal line can intersect the graph of a function in some places at more than one point, then the function involved can’t have an inverse that is also a function.

We say this function fails the horizontal line test.

Here are some examples of functions that fail the horizontal line test:

Horizontal Line Cutting or Hitting the Graph at More Than One Point

  • Graph of the parabola f\left( x \right) = {x^2} - 2.

  • Graph of absolute value function f\left( x \right) = \left| x \right|


  • Graph of semi-circle f\left( x \right) = \sqrt {7 - {x^2}}


You might also be interested in:

Vertical Line Test