# We use the vertical line test to determine if something is a function, so why do we use a horizontal line test for an inverse function opposed to the vertical line test?

##### 1 Answer

We only use the horizontal line test to determine, if the inverse of a function is truly a function. Here's why:

#### Explanation:

First, you have to ask yourself what the inverse of a function is, it's where x and y are switched, or a function that is symmetric to the original function across the line, y=x.

So, yes we use the vertical line test to determine if something is a function. What is a vertical line? Well, it's equation is x = some number, all lines where x is equal to some constant are vertical lines.

Therefore, by the definition of an inverse function, to determine if the inverse of that function is a function or not, you will the horizontal line test, or y= some number, notice how the x switched with the y...all lines where y is equal to some constant are horizontal lines.