Ctg18°ctg36°ctg54°ctg72°=
Ctg18°*ctg36°*ctg54°*ctg72°= ctg18°*ctg36°*ctg(90°-36°)*ctg(90°-18°)= ctg18°*ctg36°*tg36°*tg18°=(tg18°*ctg18°)*(tg36°*ctg36°)=1*1=1.