（1）f(x)=（cosx）^2+√3sinxcosx-1/2=1/2(1+cos2x)+√3/2sinxcosx-1/2=1/2cos2x+√3/2sin2x
=sin(2x+30°),f(x)最大值为1,此时x=30°=派/6
（2）由cos(a-b)=4/5,cos(a+b)=-3/5,0