Can You Prove the Unseen? A Challenge in Complex Analysis

[desmal]

Can you prove the next rule?

http://up.w6wup.com/up2/2006/06/22/w6w_20060622062334bcab4268.JPG

I don't Know if you see this befor becuse I But it from my mind

 

[Tide]

HINT: Write the cosine in (complex) exponential form and sum the geometric series.

 

[desmal]

yes Tide... you can do it that way
also there is anthour easy way to solve the proplem

 

[Tide]

yes Tide... you can do it that way
also there is anthour easy way to solve the proplem

Well then don't keep us in suspense - please share! :)

 

[desmal]

actully in the beginning I discover this from the complex analysis but look at this===>

http://img90.imageshack.us/img90/2028/proof9zu.gif
http://img90.imageshack.us/img90/2028/proof9zu.gif