How can you tell if a function is a one-to-one?

Question

anonymous · Answer

when you put a number in for x, you only get one answer. in y^2=x, if x is 1, then the y could be either -1 or 1. that is not a 1 to 1 function.  the typical way to test it is to use the vertical line test. if you draw a vertical line on your coordinate plane and it only intersects your line once, then it is a one to one function

anonymous · Answer

Utilize the horizontal line test to see if a function is one-to-one.  If you can draw a horizontal line so that it intersects in more than one point of the graph (such as is the case with the right hand side graph), then it is NOT one-to-one.  The graph on the left hand side is one-to-one. |dw:1315289976911:dw|