Mark M. answered 10/28/19
Retired Math prof with teaching and tutoring experience in trig.
(1 - tan(-θ)) / tan(-θ)
Since tanθ is an odd function, tan(-θ) = -tanθ
So, we have (1 + tanθ) / (-tanθ) = [(1 + sinθ / cosθ] / [-sinθ / cosθ]
= [ (cosθ + sinθ) / cosθ ] / [-sinθ / cosθ ]
= (cosθ + sinθ) / (-sinθ) = -cosθ / sinθ - sinθ / sinθ = -cotθ - 1