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.....

Printable View

- October 3rd 2009, 05:07 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..... - October 3rd 2009, 11:46 PMmr fantastic
- October 4th 2009, 02:36 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