# trig and complex numbers

• July 28th 2013, 08:23 AM
earthboy
trig and complex numbers
if n is an integer x is real, then show that :
$(1+\cos x + i\sin x )^n + (1+ \cos x-i\sin x)^n = 2^{n+1}\cos ^n {\frac{x}{2}}\cos \frac{nx}{2}$

I played around a bit with de moivre's theorem and other things...(Headbang)

• July 28th 2013, 08:58 AM
ChessTal
Re: trig and complex numbers
Quote:

Originally Posted by earthboy
if n is an integer x is real, then show that :
$(1+\cos x + i\sin x )^n + (1+ \cos x-i\sin x)^n = 2^{n+1}\cos ^n {\frac{x}{2}}\cos \frac{nx}{2}$

I played around a bit with de moivre's theorem and other things...(Headbang)

If you can't solve after a massive amount of calculations, there is always the mighty mathematical induction.(Cool)
• July 29th 2013, 04:05 AM
earthboy
Re: trig and complex numbers
Quote:

Originally Posted by ChessTal
If you can't solve after a massive amount of calculations, there is always the mighty mathematical induction.(Cool)

yes there is.....but there got be a silly neat trick here...anyone???
• July 29th 2013, 06:41 AM
BobP
Re: trig and complex numbers
Start by using the identities

$\cos 2A = 2\cos^{2}A - 1$ and $\sin 2A = 2\sin A \cos A$ so as to get everything inside the brackets in terms of half angles $x/2.$

Next, remove $2\cos(x/2)$ as a common factor from each of the brackets.

Then, write the two complex numbers in exponential form and finally use the exponential form for $\cos A.$
• August 13th 2013, 09:48 PM
ibdutt
Re: trig and complex numbers