Please help me..
Thanks,
Probably can't because this is in Pre-University Trigonometry.
@kjchauhan
My only problems with this as a "proof" is your original statement: $\displaystyle cos(n \theta) = \text{...}$. Was this given?
My other problem is your limit statement: $\displaystyle \lim_{\theta \to 0} \frac{sin ( \theta )}{ \theta } = 1$. Was this given as well?
If they are given, it looks great. If they are not given then you have some more work to do.
-Dan