tanα=2, sinα/cosα=2, sinα=2cosα,
所以(sinα)^2=4(cosα)^2=1-(cosα)^2
得到（cosα)^2=1/5.
(sinα-cosα)/(sinα+cosα)
=(sinα)^2-(cosα)^2=3(cosα)^2=3/5
sin^2α+sinαcosα
=(sin^2α+sinαcosα)/(cosα ^2 +sinα ^2)分子分母同除以cosα^2
=(tanα ^2+tanα) /(1+tanα ^2)=(4+2)/(1+4)