If cos(a) = 2/3,find the value of cos(4a)
问题描述：
If cos(a) = 2/3,find the value of cos(4a)
答
cos(4a) = 2[cos(2a)^2] -1 = 2[(2cos(a)^2 -1)^2] - 1
= 2[2*(2/3)^2-1]^2 - 1
= 2[2*4/9 - 1]^2 - 1
= 2[-1/9]^2 - 1
= 2(1/81) - 1
= 2/81 - 1
= -79/81