1、
f(x)=cos²a-sinacosa
=1/2*(1+cos2a)-1/2*sin2a
=-(√2/2)sin(2a-π/4)+1/2
系数小于0
所以f(x)减则sin增
sinx增区间是(2k-π/2,2kπ+π/2)
2k-π/2