I am trying to prove that cos(pi/5)= (1+sqrt(5))/4

I tried finding the 5th roots of unity and then just using the real part cis(pi/5) but then i dont know how to get it = (1+sqrt(5))/4.

I am stuck somewhere here.

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- February 10th 2009, 09:49 PMkayla09[SOLVED] Complex Numbers- cos ( pi/5)
I am trying to prove that cos(pi/5)= (1+sqrt(5))/4

I tried finding the 5th roots of unity and then just using the real part cis(pi/5) but then i dont know how to get it = (1+sqrt(5))/4.

I am stuck somewhere here. - February 11th 2009, 12:19 AMnzmathman
- February 11th 2009, 08:18 PMkayla09
i am still confused though. that website is for sin(pi/5) i let x=cos(theta) and now i have (x+y)^5 and i expanded it out. but now i am still stuck.

- February 11th 2009, 11:11 PMGrandadCos pi/5
Hello kayla09

nzmathman has shown you how to find . Now use

, taking the positive square root.

Grandad

- February 12th 2009, 04:44 AMkayla09
Thanks!!!