sinα=5/13,且a∈（π/2,π）cosα=-12/13cos2α=1-2sin^2α=1-2*(5/13)^2=1-50/169=119/169 sin2α=2sinαcosα=-120/169 cosα=1-2sin^2α/2=-12/13sin^2α/2=25/26 sinα=5/13,且a∈（π/2,π）s...