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sin54°Cos72°=?

解: sin54°cos72° =cos36°sin18° =cos36°sin18°cos18°/cos18° =cos36°×1/2sin36°/cos18° =1/4sin72°/cos18° =1/4

解答: cos72°-cos36° =cos(54°+18°)-cos(54°-18°) =(cos54°cos18°-sin54°sin18°)-(cos54°cos18°+sin54°sin18°) =-2sin54°sin18° =-2cos36°cos72° =-2sin36°cos36°cos72°/sin36° =-sin72°cos72°/sin36° =-(1/2)sin144°/sin36° =-(1/2)sin(180°-3...

cos72°-cos36° =cos(54+18)-cos(54-18) =[cos54cos18-sin54sin18]-[cos54cos18+sin54sin18] =-2sin54sin18 (sin54=cos36) =-2cos36sin18 =-2cos36sin18cos18/cos18 =-cos36sin36/cos18 =-sin72/(2cos18) =-sin72/(2sin72) =-1/2

cos36-cos72 =cos(54-18)-cos(54+18) =cos54cos18+sin54sin18-[cos54cos18-sin54sin18] =2sin54sin18 =-2sin54sin(-18) =2sin54sin18 =2cos36sin18 =2cos36sin18cos18/cos18 =cos36sin36/cos18 =sin72/(2cos18) =sin72/(2sin72)=1/2

解答: 利用三倍角公式 sin3a=4sinasin(60°+a)sin(60°-a) 证明如下: sin3a=3sina-4sin^3a =4sina(3/4-sin^2a) =4sina[(√3/2)^2-sin^2a] =4sina(sin^260°-sin^2a) =4sina(sin60°+sina)(sin60°-sina) =4sina*2sin[(60+a)/2]cos[(60°-a)/2]*2sin[(...

1)先解出sin18,设sin18=x. 因为cos36=sin54,注意36=18*2,54=18*3,由2 倍角和3倍角公式,得 1-2x^2=3x-4x^3,即4x^3-2x^2-3x+1=(x-1)(4x^2+2x-1)=0, 由于sin18≠1,解4x^2+2x-1=0得x=(√5-1)/4=sin18 2)x=cos36-cos72=1-2(sin18)^2-sin18=0.5 ...

cos36-cos72 =cos(54-18)-cos(54+18) =cos54cos18+sin54sin18-[cos54cos18-sin54sin18] =2sin54sin18 =-2sin54sin(-18) =2sin54sin18 =2cos36sin18 =2cos36sin18cos18/cos18 =cos36sin36/cos18 =sin72/(2cos18) =sin72/(2sin72)=1/2

(sin37°=0.6 cos37°=0.8) 浮衍2097 | ...二号滚_蛋502 采纳率:72% 擅长: 暂未定制 为您...54 2014-01-16 如图所示,斜面倾角为θ=370,在斜面...

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