f(x)=sin（x+6分之π）+sin（x-6分之π）+cosx+a
=2sinxcosπ/6+cosx+a
=√3sinx+cosx+a
=2sin(x+π/6)+a
最大值为2+a=1 a=-1
f（x）≥0
2sin(x+π/6)-1>=0
sin(x+π/6)>=1/2
2kπ+π/6