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Cos乘以Cos

cos3x=cos(2x+x)=cos2xsinx-cosxsin2x=cos^2xsinx-sinx-2sinxcos^2x=-sinx-sinxcos^2x 再乘以cosx得-sinxcosx-sinxcos^3=-sinxcosx(1+cos^2)

cos(x+y)=cosxcosy-sinxsiny

cosα乘以cosβ=2分之1乘以【cos(α+β)+cos(α-β)】=(cosαcosβ-sinαsinβ+cosαcosβ+sinαsinβ)/2=(2cosαcosβ)/2=cosαcosβ;所以左边等于右边

cosx * cos(x+α)根据积化和差公式,原式 = 1/2{cos(x+x+α)+cos(x+α-x)}= (1/2)cos(2x+α) + (1/2)cosα

你比方说COS50*TAN50=COS50*(SIN50/COS50),不就可以约下去了么,就剩下了SIN50带角度的SIN351*SIN99=(这里的角度每360是个循环)1.SIN351=SIN(351-360)=SIN(-9)=-SIN9SIN99=SIN(90+9)这里奇数个90的倍数就要SIN变COS了=COS9所以整个就=-SIN9*COS9=-1/2SIN18

因为两个向量要共起点.现在AC和CB的起点分别是A和C 所一呀平移到共起点 所以就是120度的补交望采纳,谢谢

解法一: 直接查函数表或用计算器: cosπ/5*cos2π/5=cos36°*cos72°. =0.8090*0.3090 =0.2499. ∴原式=0.25. 解法二:应用积化和差公式: (1/2)*2cosπ/5*cos2π/5=(1/2)[cos(π/5+2π/5)+cos(π/5-2π/5)]. =(1/2)[cos3π/5+cos(-π/5] . =(1/2)(cos108+cos36). =(1/2)(-cos72+cos36) =(1/2)(-0.3090+0.8090) =(1/2)*0.5 ∴原式=0.25.

是求值吗? 若是这样,则方法如下:原式=cos(2π/14)cos(4π/14)cos(6π/14) =2sin(2π/14)cos(2π/14)cos(4π/14)cos(6π/14)/[2sin(2π/14)] =sin(4π/14)cos(4π/14)cos(6π/14)/[2sin(π/7)] =2sin(4π/14)cos(4π/14)cos(6π/14)/[4sin(π/7)] =sin(8π/14)cos(6π/14

cos72cos36=(2sin36cos36cos72)/(2sin36) =(sin72cos72)/(2sin36) =(2sin72cos72)/(4sin36) =sin144/(4sin36) =sin36/(4sin36) =1/4

你好,很高兴回答你的问题cos36°cos72°=(2sin36°cos36°cos72°)/(2sin36°)=sin72°cos72°/(2sin36°)=(sin144°/2)/(2sin36°)(144°+36°=180°,sin144°=sin36°)=1/4

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