已知函数f(x)=1/2cos2x+根号3sinxcosx-2cos^2x

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＝0
f(a)=-1/5
则,sin(2a-π/6)-1＝-1/5
sin(2a-π/6)=4/5
所以,cos(2a+π/3)
＝cos(2a-π/6＋π/2)
＝-sin(2a-π/6)
=-4/5
f(a)=-1/5
则,sin(2a-π/6)-1＝-1/5
cos(2a+π/3)=-4/5
π/3