证明cos 2x cos x=1/2(cos x+cos 3x)

证明cos 2x cos x=1/2(cos x+cos 3x) 怎么证明2(1-sin x)(1+cos x)=(1-sin x+cos x)^2 证明COS（X+Y）COS（X-Y）=COS^2X-SIN^2Y 证明cos(x+y)cos(x-y)= cos^2(x)-sin^2(y) (cos X-1)^2+sin^2X=2-2cos X 证明题(要过程) 证明：sin^2x+cos^2x=1 Cos4X=8cos^4X-8cos^2X+1 请问怎么证明, 证明sin^2(x)+cos^2(x+30)+sin(x)cos(x+30)=3/4 证明sin^4x-cos^4x=sin^2x-cos^2x 证明f(x)=cos^2x+cos^2(x+∏/3)+cos^2(x-∏/3)是常数函数 证明cos(arcsinx)=根号（1-x^2） 一道三角函数证明题sin^4 X+cos^4 X=1-2sin^2 X cos^2 X 证明1-sin^6x-cos^2x/1-sin^4x-cos^4x=3/2 证明:函数f(x)=cos^2+cos^2(x+π/3)+cos^2(x-π/3)是常数函数 2（cos x）^2=1+cos 2x, 证明1-2sinxcosx/cos^2x-sin^2x=cos^2x-sin^2/1+2sinxcosx y=(cos x-2)/(cos x-1)的值域 证明：当x->0时,1-cos～x^2/2