Cos2A=CosA^2-SinA^2=1-2SinA^2=2CosA^2-1
(注:SinA^2 是sinA的平方 sin2(A) )
sin3α=4sinα·sin(π/3+α)sin(π/3-α)
cos3α=4cosα·cos(π/3+α)cos(π/3-α)
tan3a = tan a · tan(π/3+a)· tan(π/3-a)
Asinα+Bcosα=(A^2+B^2)^(1/2)sin(α+t),其中
Asinα+Bcosα=(A^2+B^2)^(1/2)cos(α-t),tant=A/B
sin^2(α)=(1-cos(2α))/2=versin(2α)/2
cos^2(α)=(1+cos(2α))/2=covers(2α)/2
tan^2(α)=(1-cos(2α))/(1+cos(2α))
=2sina(1-sin2a)+(1-2sin2a)sina
=(2cos2a-1)cosa-2(1-sin2a)cosa
=4sina(sin60°+sina)(sin60°-sina)
=4sina.2sin[(60+a)/2]cos[(60°-a)/2].2sin[(60°-a)/2]cos[(60°-a)/2]
=4sinasin(60°+a)sin(60°-a)
=4cosa(cosa+cos30°)(cosa-cos30°)
=4cosa.2cos[(a+30°)/2]cos[(a-30°)/2].{-2sin[(a+30°)/2]sin[(a-30°)/2]}
=-4cosasin(a+30°)sin(a-30°)
=-4cosasin[90°-(60°-a)]sin[-90°+(60°+a)]
=-4cosacos(60°-a)[-cos(60°+a)]
=4cosacos(60°-a)cos(60°+a)
tan3a=tanatan(60°-a)tan(60°+a)