sin(α+π)=-3/5 -sinα=-3/5
sinα=3/5 α∈(π/2,π)
cosα=-4/5
cos2α=1-2sin^2α=7/25
sin2α=2sinαcosα=-24/25
tan2α=sin2α/cos2α=-24/7
(其中/为分数线,sin^2α为sinα的平方)