What is the horizontal line test?

In January of this year, we have discussed about the vertical line test. We have learned that if a vertical line intersect a graph more than once, then that graph is not a function. In this post, we learn about the horizontal line test and its relation to inverse functions.

Suppose we have a function f. Then, we input x and call the output f(x). If we do things backward, suppose we have the number y which is in the range of f. What value should we input to f to get y? Let’s have a more specific example.

Suppose we have the function f(x) = x^2. And we choose the number y = 9 which is in the range of f. What number should we input in f to get 9? Well, we will have two numbers, those are 3 and -3. » Read more

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