How i can sove the problem given below:
(Cosx-Cos2x)/1-cosx

What do you mean by “solve this?” Do you mean simplify?

What you wrote is this:

\frac{\cos x - \cos (2x)}{1 - \cos x }

I wonder if what you actually meant is this:

\frac{\cos x - \cos ^2x}{1 - \cos x }

Please clarify.