cos(-61π/12)=cos(-6π+11π/12)=cos(11π/12)=cos(3π/12+8π/12)=cos(π/4+2π/3)=cos(π/4)cos(2π/3)-sin(π/4)sin(2π/3)=√2/2*(-1/2-√3/2)=-√2/4-√6/4