[block](2cos^2(Pi/10))/(ctg(11Pi/10)*sin(Pi/5))[/block]
Так как ctg(11π/10)=ctg(π+(π/10))=ctg(π/10)=cos(π/10)/sin(π/10) и 2cos(π/10)*sin(π/10)=sin(π/5) то 2cos^2(π/10)/(ctg(11π/10)*sin(π/5))= =(2cos^2(π/10)*sin(π/10))/(cos(π/10)*sin(π/5))= =2cos(π/10)*sin(π/10)/sin(π/5)=sin(π/5)/sin(π/5)=1