1. ## Double 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.....

2. Originally Posted by anonymous_maths
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.....
It can't be done using that identity. Either the wrong identity has been given or the wrong angle to found has been given.

3. 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