f(-11/6)=sin(-11/6π)
=sin(π/6-2π)
=-sin(π/6)
=-1/2
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
=-5/2
f（-11\6）+f(11/6)=-1/2-5/2=-3