# One to one function

I am having trouble with this study question for my final:

A function from the real numbers to the real numbers is one to one on an interval I if it is strictly increasing on that interval.

I am not quite sure how to prove it, I know that the use of strictly increasing is important as far as if x1 < x2, then f(x1) < f(x2). A hint I was told to use is contradiction.