kbys.net
当前位置:首页 >> Cos72度Cos72度等于多少 >>

Cos72度Cos72度等于多少

cos(72°)=0.30901699437495

请采纳

根据正弦定理:sin(A+B)=sinAcosB+cosAsinB即sin72°cos18°+cos72°sin18° =sin(72°+18°) =sin90° =1

根据正弦定理:sin(A+B)=sinAcosB+cosAsinB即sin18cos72+sin72cos18=sin(18+72)=1

额 底边不是6 这是黄金三角形 黄金比是(根号5 -1)/2 就是腰长和底长的比 你可以算算

如果我的答案对您有用,请尽快采纳我的答案,谢谢!∵|AB|=√(cos²18°+cos²72°)=√(cos²18°+sin²18°)=1, |BC|=√(4cos²63°+4cos²27°)=√(4sin²63°+4cos²27°)=2, cos∠ABC=BA·BC/|BA||BC|=2sin45°/2=√2/2, ∴∠ABC=45°, ∴面积=√2/2。

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

由cos54°=sin36°即倍角公式,得 4(cos18°)^3-3cos18°=2sin18°cos18°,cos18°≠0, ∴4(cos18°)^2-3=2sin18°, ∴4(sin18°)^2+2sin18°-1=0,sin18°>0, 解得sin18°=(-1+√5)/4, ∴cos18°=√[1-(sin18°)^2]=√(10+2√5)/4, ctg72° =tan18° =sin18°/cos18° ...

sin42度sin72度+cos42度cos72度 =cos72度cos42度+sin72度sin42度 =cos(72度-42度) =cos30度 =sin60度 答案是A

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

网站首页 | 网站地图
All rights reserved Powered by www.kbys.net
copyright ©right 2010-2021。
内容来自网络,如有侵犯请联系客服。zhit325@qq.com