(1)sin2α=2sinacosa/(sin²a+cos²a) ,(上下除以cos²a)=2tana/(tan²a+1 ) =-3/5(2)∵α∈[0,π],2α∈[0,2π],sin2α=-3/5,∴ 2α∈[π,2π],cos2a=±4/5β∈【0,π】,cosβ=√5/5∴β∈【0,π/2...