Use the identity cos(2x)=2cos^2(x)-1 to form a quadratic equation in terms of cos(pi/5), hence find the exact value of cos(pi/5).

Any help please? I tried sub (pi/5) in but doesn't seemed to work.... Thanks.....

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- Oct 3rd 2009, 04:07 PManonymous_mathsDouble angle identities
Use the identity cos(2x)=2cos^2(x)-1 to form a quadratic equation in terms of cos(pi/5), hence find the exact value of cos(pi/5).

Any help please? I tried sub (pi/5) in but doesn't seemed to work.... Thanks..... - Oct 3rd 2009, 10:46 PMmr fantastic
- Oct 4th 2009, 01:36 AMpacman
your supposed identity is unknown to me . . . .

this may help, Surds-prove a formula for sin36? - Yahoo! Answers

steiner1745 says something about that identity but is not. .

cos 36 = (1 + √5)/4