11/6>0
f(11/6)=f(11/6-1)-1=f(5/6)-1=f(5/6-1)-1-1=f(-1/6)-2=sin(-π/6)-2=-1/2-2=-3/2
f(-11/6)=sin(-11π/6)=1/2
所以原式=1/2-3/2=-1