cos2t=cos(t+t)根据公式cos(t+t)=costcost-sintsint=cos²t-sin²t
公式cos(a+b)=cosa*cosb-sina*sinbcos(a-b)=cosa*cosb+sina*sinba=2t, b=t
cost=cos(2t-t)=cos2tcost+sin2tsint
