>>205
∠CnBnAn+1 = (π-∠Bn)/2
∠BnCnAn+1 = (π-∠Cn)/2

∠An+1 = π - (CnBnAn+1 + ∠BnCnAn+1)
=(∠Bn + ∠Cn)/2
=π/2−1/2∠Ａn