Proving One-to-One Function Strictly Increasing on Interval I

[staw_jo]

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.

Any help please!

 

[VeeEight]

Question is answered here: https://www.physicsforums.com/showthread.php?t=409073