sinα＝-3/5=>cosα=4/5, tanα=-3/4sin2α = 2sinα.cosα = 2(-3/5)(4/5) =-24/25cos2α = 2(cosα)^2 -1 = 2(4/5)^2 -1 = 7/25tan2α = 2tanα/(1- (tanα)^2) = 2(-3/4) /[ 1- (-3/4)^2 ] = -(3/2)/ (7/16) = -24/7
如图所示
