cos2x= (cosx)^2 - (sinx)^2
= 2(cosx)^2-1
(cosx)^2 = (1+cos2x)/2为什么cos2x= (cosx)^2 - (sinx)^2cos(A+B) = cosAcosB - sinAsinBA=B = xcos2x = (cosx)^2 - (sinx)^2