要用到的公式：Cos2a=2Cos^2(a)-1 sin2A=2sinA·cosA tan2A=[2tanA]/[1-tan^2(A)]2+2cos2α=sin2α2+2[2Cos^2(α)-1]=2sinα·cosα2+4Cos^2(α)-2=2sinα·cosα4Cos^2(α)=2sinα·cosα同除以2Cos^2(α)得2=tanα...