由tan(45°+α)=(tan45°+tanα)/(1-tan45°*tanα)=(1+tanα)/(1-tanα)=3解得：tanα=1/2则sin2α-2cosα^2=(2sinα*cosα-2cosα^2)/1=(2sinα*cosα-2cosα^2)/(sinα^2+cosα^2)由tanα=sinα/cosα有意义,所以...