已知tanα=3,求
①3/4*(sinα)^2+1/2*(cosα)^2'
②sinα·cosα
tana=3 sina=3/√ 10 cosa=1 /√ 10
(sina)^2=9/10 ( cosa)^2=1/10
代入① 3/4*(sinα)^2+1/2*(cosα)^2'=29/40
②sinα·cosα=3、10