sin36°=cos54°用倍角公式展开得2sin18°cos18°=4(cos18°)^3-3cos18°,2sin18°=4(cos18°)^2-3,4(sin18°)^2+2sin18°-1=0,sin18°=(√5-1)/4.由二倍角余弦公式得cos36°=1-2(sin18°)^2=（√5+1)/4....