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.....
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