1+ctg²(π/5) =
2*cos12°*cos48° =
Sin40°-Sin18° =

1+ctg²(π/5)=1/sin²(π/5)2cos12*cos48=2*1/2(cos12-48)+cos(12+48))=cos36+cos60=cos36+1/2sin40-sin18=2sin(40-18)/2*cos(40+18)/2=2sin22*cos29