cosx^2-sinx^2
采用降幂公式得：cosx^2=1/2(1+cos2x) sinx^2=1/2(1-cos2x)
所以 cosx^2-sinx^2=1/2(1+cos2x) -1/2(1-cos2x)=1/2(1+cos2x-1+cos2x)=cos2x