Verify the identity .
cot (treta-pi/2) = -tan treta
first off, it's theta, not treta!
cot(θ-π/2) = (1+cot(θ)cot(π/2))/(cot(π/2)-cot(θ))
= -1/cotθ
= tanθ
cot (treta-pi/2) = -tan treta
cot(θ-π/2) = (1+cot(θ)cot(π/2))/(cot(π/2)-cot(θ))
= -1/cotθ
= tanθ