3 14-3 7cos^2π 12
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解2cos²π/12-1=cos(2×π/12)=cos(π/6)=√3/2用到二倍角公式cos2a=2cos²a-1
1、α在第四象限,sinα=-5/13,cos(α+π/4)=√2/2cosα-√2/2sinα=17√2/262、原式=(sinα)^2+(sinβ)^2-2sinαsinβ+(cosα)^2+(c
sinπ/24*cosπ/24*sinπ/12=1/2(2sinπ/24*cosπ/24)sinπ/12=1/2(sinπ/12)^2=1/4(1-cosπ/6)=1/4(1-√3/2)
cos(π/12)*cos(5π/12)=cos(π/12)*cos(π/2-π/12)=cos(π/12)*sin(π/12)=(1/2)*sin(π/6)=(1/2)*(1/2)=1/4
根据公式:COS^2a=(1+COS2a)/2a=(X+π/12)说句不好听的,你还是基本知识没掌握好,不会活用知识.希望你多背多看,看清题,把公式活用.
cos(a+5π/12)cos(a+π/6)+cos(π/12-a)cos(π/3-a)=sin(π/2-(a+5π/12))cos(a+π/6)+cos(π/12-a)sin(π/2-(π/3-a)
因为cos(α)=cos((2α+β)-(α+β))=cos(2α+β)cos(α+β)-sin(2α+β)sin(α+β),又cos(α+β)=12/13,cos(2α+β)=3/5且β∈(0,π/
cos^2(π/3-a)+cos^2(π/6+a)=cos^2(π/3-a)+sin^2(π/3-a)=1再问:为什么cos^2(π/6+a)=sin^2(π/3-a)啊小弟我没学过╮(╯_╰)╭再答
cos2π=cos(2π+0)=cos0=1
π/12就是15°cos15°=cos(45°-30°)然后用诱导公式计算就可以算出cos15°然后平方-1/2就好了不用我一一计算了吧!要自己动手计算一下啊要不是考试的时候很作难的!住你学习愉快!
(1)cos(-165°)=-cos15°=0.9659(2)cos(7π/12)=-cos(5π/12)=0.2588(3)cos(-61π/12)=cos(61π/12)=-cos(π/12)=-
∵5π12+θ+π12-θ=π2,∴π12-θ=π2-(5π12+θ),∴sin(π12-θ)=sin[π2-(5π12+θ]=cos(5π12+θ),∴sin(π12-θ)=cos(5π12+θ)=
不能直接使用,只有α是特殊角才可以-cos(2π/3+2α)=cos2α化简cos(2π/3+2α)=-cos2α-1/2cos2a-√3/2sin2a=-cos2a1/2cos2a-√3/2sin2
=cosα+cos2π/3*cosα-sin2π/3*sinα+cos4π/3*cosα-sin4π/3*sinα=cosα-1/2cosα-(根号3)/2sinα-1/2cosα+(根号3)/2si
θ∈(0,π/2)sinθcosθθ∈(0,π/2)cosθ∈(0,1)又sinθ
∵−π<α<−π2∴−7π12<α+ 5π12<−π12∵cos(5π12+α)=13∴sin(α+5π12)=−223∵(5π12+α)+(π12−α)=π2,∴cos(π12−α)=co
cos(π/6)=√3/2它的平方就是3/49
cos^2θ+cos^2(θ+2π/3)+cos^2(θ-2π/3)=(1+cos2θ)/2+[1+cos(2θ+4π/3)]/2+[1+cos(2θ-4π/3)]/2=3/2+[cos2θ+cos(
cos(-2/3)π=-0.5