i know for a equation to be 1-1 they have to pass the horizontal line test. so say for example (-8 + x) ^4 i know this is not a one to one because it gives me a parabola. The problem that I am having is being able to explain in a equation. MY SOLUTION First derive the equation so that i know where it is increasing/decreasing 4(-8+x)^3 so i take -8+x>0 is increasing x>8 <----- im sure thats correct now to find it decreasing you flip the sign and i take -8+x<0 is decreasing??? but then i also get x<8 What I'm i doing wrong?? So if I am doing this right then that means that for a function to be one to one.. It either has to be increasing OR decreasing?? but cannot be both?