直线xcosθ＋ysinθ＋1＝0的斜率为－cotθ,直线的倾斜角为α
∴tanα＝－cotθ＝－tan(π/2－θ)＝tan(θ－π/2)
∵θ∈(π/2,π)
∴α＝θ－π/2