∵关于x的二次方程有两个相等的实根
∴△=b^2 - 4ac =(-6sinθ)^2 - 4*1*tanθ=36(sinθ)^2 - 4tanθ=0
9(sinθ)^2 = (sinθ)/(cosθ)
sinθ*cosθ=1/9
(sinθ-cosθ)^2 =(sinθ)^2 - 2sinθ*cosθ + (cosθ)^2 =1 - 2/9 =7/9
∵π/2