Jika cos alfa = akar5/5, nilai cotan (phi/2 – alfa)

Cos a = √5 /5
cotan (π/2 – a) = tan a

cos a = √5 /5
samping = √5
miring = 5
depan = √(5² – 5) = 2√5

tan a = depan/samping
tan a = (2√5)/√5
tan a = 2