if you google
(1 + tan 21°)(1 + tan 28°)(1 + tan 24°)(1 + tan 17°) = 4
you will find several proofs
(1 + tan 21 degrees)(1 + tan 28 degrees) * (1 + tan 24 degrees) * (1 + tan 17 degrees) = 4
2 answers
Compound angel