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Cos72度*Cos36度

cos72°cos36° =2sin36°cos72°cos36°/2sin36° (分子分母同时乘以2sin36°) =sin72°cos72°/2sin36° (2sin a * cos a=sin2a,所以2sin36°cos36°=sin72°) =2sin72°cos72°/4sin36° (分子分母同时乘以2) =sin144°/4sin36° (2sin a * cos a=sin2a,...

解答: 不知你有没有学过和差化积公式, 以下按没学过处理(如果学过,可以简化以下步骤) cos72°—cos36° =cos(54°+18°)-cos(54°-18°) =cos54°cos18°-sin54°sin18°-(cos54°cos18°+sin54°sin18°) =-2sin54°sin18° =-2cos36°cos72° =-2sin36°cos36...

M=cos36*cos72 N=sin36*sin72 M*N=1/4*(sin72*sin144)=1/4*(sin72*sin36) M*N=1/4*(sin72*sin36)/N=1/4*(sin72*sin36)/*(sin72*sin36) =M=cos36*cos72=1/4 不懂发消息问我。 祝你学习进步!

cos36cos72=(2sin36cos36)cos72/(2sin36)=(sin72cos72)/(2sin36)=(2sin72cos72)/(4sin36)=sin144/(4sin36)=sin36/(4sin36)=1/4

cos72—cos36 =cos(54+18)-cos(54-18) =[cos54cos18-sin54sin18]-cos54cos18+sin54sin18 =-2sin54sin18 =-2cos36sin18 =-2cos36sin18cos18/cos18 =-cos36sin36/cos18 =-sin72/(2cos18) =-sin72/(2sin72) =-1/2

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