tanα=3,α属于(π/4,π/2)
sinα = 3/√10
cosα = 1/√10
sin2α
=2sinαcosα
= 2(3/√10)(1/√10)
=3/5
cos2α
= (cosα)^2 - (sinα)^2
= 1/10 - 9/10
= -4/5
tan2α
= 2tanα/(1-(tanα)^2)
=2(3)/(1-9)
=-3/4