cos连乘公式 多个cos相乘求极限

原式=2sin20cos20cos40cos80/2sin20 =sin40cos40cos80/2sin20 =2sin40cos40cos80/4sin20 =sin80cos80/4sin20 =2in80cos80/8sin20 =sin160/8sin20 =sin(180-20)/8sin20 =sin20/8sin20 =1/8

1/2

tan=sin/cos, cot=1/tan =cos/sin, sin=tan*cos=cos/cot=1/csc,cos=sin/tan=sin*cot=1/sec 依次代换就行了!

cosA cosB=0.5cos(A+B)+0.5cos(A-B)

cos30=2分之根号3,cos60=2分之1,cos90=0,cos0=1,cos(180-x)=-cosx,cos(90-x)=sinx ,cos2x=(cosx)平方-(sinx)平方=2(cosx)平方-1=1-2(sinx)平方

平立角,竖立角,侧立角,C为侧立角

原式=sina·cosa·cos2a·cos4a············cos2^na/sina =sin2a·cos2a·cos4a···········cos2^na/2sina =sin4a·cos4a·········cos2^na/4sina =sin2^(n+1)a/2^(n+1)sina

这是余弦,有定值的,不是算出来的

cos(α±β)=cosαcosβ±sinαsinβ

你只需知道:cosxcosy-sinxsiny=cos(x+y) cosxcosy+sinxsiny=cos(x-y) 上下相加:2*cosxcosy=cos(x+y)+cos(x-y) 然后x=b+a, y=c+a就可以得到你的式子.