If π<θ<2π, then 1+cosθ1−cosθ is equal to
cosecθ+cotθ
cosecθ−cotθ
−cosecθ+cotθ
−cosecθ−cotθ
we have,
1+cosθ1−cosθ=(1+cosθ)21−cos2θ⇒ 1+cosθ1−cosθ=1+cosθ|sinθ|⇒ 1+cosθ1−cosθ=1+cosθ−sinθ [∵π<θ<2π⇒sinθ<0]
⇒ 1+cosθ1−cosθ=−cosecθ−cotθ