cos(a+π/3)=-5/13,sin(2π/3+b)=3/5 sin (a+π/3)=12/13 ,cos(2π/3+b)=-4/5
sin(a+b)=sin[(a+π/3)+(2π/3+b)-π]
=-sin[(a+π/3)+(2π/3+b)]=-[(12/13)(-4/5)+(-5/13)(3/5)]=63/65