Originally Posted by

**Kakariki** Hey, I have two problems here that I really don't know how to start. Hopefully someone can help me out!

**Question**

Prove that $\displaystyle cos(sin^{-1}x) = \sqrt{1-x^2} $

**Solution **

I don't know where to start with this problem. I know that $\displaystyle sin^{-1}x = y -> siny = x $. I don't know how to apply this to this equation. A similar problem in the examples of my text is: "simplify: $\displaystyle cos(tan^{-1}x) $.

To simplify this the text first let y = $\displaystyle tan^{-1}x $. Then it jumps to $\displaystyle sec^2y = 1 + tan^2y = 1 + x^2 $. I don't understand this step in their example and this may prove to be the problem I am having.

I hope I am making sense here! I have another problem, but I figure once I get help with this one I can force my way though the second. Please help!