f(x)=1/2cos2x+根号3/2sin2x-1-cos2x =√3/2 ×sin2x－1/2 ×cos2x －1 ＝sin(2x-π/6) －1当sin(2x-π/6)＝1时f(x)有最大值＝0f(a)=-1/5则,sin(2a-π/6)-1＝-1/5sin(2a-π/6)=4/5所以,cos(2a+π/3)＝cos(2a...