sin(π/3+a)=-根号(1-1/9)=-2根号2/3
sin(2∏/3-a)+cos(a-2∏/3)
＝sin(π-（π/3+a))+cos(-π+（a+π/3))
=sin(π/3+a)-cos(a+π/3)
=-2根号2/3-1/3
=-(2根号2+1）/3