cos72°-cos36°=cos(54+18)-cos(54-18)=[cos54cos18-sin54sin18]-[cos54cos18+sin54sin18]=-2sin54sin18 (sin54=cos36)=-2cos36sin18 =-2cos36sin18cos18/cos18 =-cos36sin36/cos18 =-sin72/(2cos18) =-sin72/(2sin...