由象限范围可知sinα和cosα同号,则tanα大于0.不妨设α在第一象限,则sinα大于0,cosα大于0.2sinαcosα=(sinα^2+cosα^2)-(sinα-cosα)^2=1-(sinα-cosα)^2(sinα+cosα)^2=(sinα-cosα)^2+4sinαcosα=2-(sin...