по формулам приведения:
сos(61π/12)=cos((60π/12) +(π/12))=cos(5π+(π/12))= [b]-cos(π/12)[/b]
и по формуле синуса двойного угла
2*sin(π/12)*cos(π/12)=sin(π/6)= [b]1/2[/b],
то
4/(sin(π/12)·cos(61π/12))= 4/(-sin(π/12)·cos(π/12))=
=-8/(2*sin(π/12)*cos(π/12))=-8/sin(π/6)= -8/(1/2)=-16
О т в е т. [b]-16[/b]