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CospAi

cosπ/4=cos45°=(根号2)/2 特殊角比如30°、45°、60°、90°等的三角函数值要记住以便解题。

cos(θ+π/4)=(-7√2)/26,θ∈(π,3π/2) cosθcos(π/4)-sinθsin(π/4)=-7√2/26 (√2/2)cosθ-(√2/2)sinθ=-7√2/26 cosθ-sinθ=-7/13 又因为cos78;θ+sin78;θ=1 -2cosθsinθ=(cosθ-sinθ)78;-(cos78;θ+sin78;θ) (cosθ+sinθ)&...

角色扮演

利用二倍角公式:sin2θ=2sinθcosθ cos(π/5)*cos(2π/5) =2sin(π/5)*cos(π/5)*cos(2π/5)/2sin(π/5) =sin(2π/5)*cos(2π/5)/2sin(π/5) =sin(4π/5)/4sin(π/5) =sin(π/5)/4sin(π/5) =1/4

sin(π/10)·cos(π/5) =[2sin(π/10)cos(π/10)·cos(π/5)]/[2cos(π/10)] =[2sin(π/5)cos(π/5)]/[4cos(π/10)] =[sin(2π/5)]/[4cos(π/10)] =[cos(π/2-2π/5)]/[4cos(π/10)] =[cos(π/10)]/[4cos(π/10)] =1/4。

-cos(pai+x) = cos(x)

解应该是+(or-)cos(pi/8)+(or-)i sin(pi/8) 也就是说你的结果,不是该方程的解。 注意 你的解中没有虚部,所以他的四次方也不会是一个纯虚数。

√3/2

解cos(π/17)cos(2π/17)cos(4π/17)cos(8π/17) =2sin(π/17)cos(π/17)cos(2π/17)cos(4π/17)cos(8π/17)/2sin(π/17) =sin(2π/17)cos(2π/17)cos(4π/17)cos(8π/17)/2sin(π/17) =2sin(2π/17)cos(2π/17)cos(4π/17)cos(8π/17)/2×2sin(π/17) =sin(4π/17)cos...

cos(-7pai/6) =cos(-pai-pai/6) =cos(pai/2x(-2)-pai/6) -2属于偶数集,-2是偶数, 则=cos(-pai/6)。 符号看向西安, -pai/6堪称是一个锐角,再第一象限,然后该交-pai,顺时针旋转180度,到第三项西安,cos

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