f(a)=sin(a-π/6)=3/5
f(b)=sin(b-π/6)=12/13
cos(a-π/6)=4/5
cos(b-π/6)=5/13
sin(a-b)=sin[(a-π/6)-(b-π/6)]
= -33/65
cos(a-b)=56/65
f(a-b)=sin(a-b)cosπ/6-cos(a-b)sinπ/6
=-(33√3+56)/130