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Cos36°+Cos72°=______

第一步,分子分母同时乘2sin36。第二部,由二倍角公式得2sin36cos36=sin72。第三步,分子分母同时乘以2。第四步,由二倍角公式2sin72cos36=sin144。第五步,sin144=sin(180-36)=sin36。最后约分得出结果

=(sin36*cos72*cos36)/sin36=(1/2sin72*cos72)/sin36=(1/4sin36)/sin36=1/4

cos36-cos72 解1. Cos36-cos72 =(sin36cos36-sin36cos72)/sin36 =-(sin108-sin36-sin72)/2sin36 =-(sin72-sin36-sin72)/2sin36 =sin36/2sin36 =1/2 解2. cos36-cos72 =cos(54-18)-cos(54+18) =cos54cos18+sin54sin18-[cos54cos18-sin54sin18] =2...

做等腰三角形ABC,其中∠A=36° 易得∠B=∠C=72° 过A做AD⊥BC于D,∵∠B=∠C ∴BC = 2BD cos72°=cos∠B=BD/AB=BC/2AB 过C做∠C的角平分线交AB于E点. ∠ACE=1/2∠C=36°=∠A ∴CE=AE ∵∠BEC是△AEC的外角 ∴∠BEC=∠A+∠ACE=72°=∠B ∴CE=BC ∵∠ACE=∠A,∠BEC=∠B ∴△ABC∽△CEB ∴A...

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...

求cos36有很多方法,即使你是初中生也可以看懂,我也两种 1。相似三角形法。 做三角形ABC,∠A=108,∠B=∠C=36,过A作AD交BC于D,使∠ADC=72 那么∠ADC=∠DAC=72,∠B=∠ADB=36,所以AD=DB,DC=AC 令AB=x,BD=a,因为三角形ABD∽三角形ABC,所以AB^2=BD*BC x^2=...

=cos36°+cos72°+cos(180°-72°)+cos(180°-36°) =cos36°+cos72°-cos72°-cos36° =0

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

cos18*cos36*cos72*cos144 上下同乘以16sin18 =16*sin18*cos18*cos36*cos72*cos144/(16sin18) 反复运用2sinacosa=sin2a =8sin36cos36cos72cos144/(16sin18) =4sin72cos72cos144/(16sin18) =.. =sin288/(16sin18) 因为:sin(360-a)=-sina =-sin72...

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