k=-cosθ/sinθ= [-sin(π/2-θ)]/[cos(π/2-θ)
=[sin(θ-π/2)]/cos(θ-π/2)=tan(θ-π/2)
而θ-π/2∈（π/2,π）由倾斜角的定义可知：
直线倾斜角为θ-π/2