α=180-∠ABC-∠ACB
β=180-1/2∠ABC-∠BCE
∠BCE=∠ACB+(180-∠ACB)/2=90+∠ACB/2
β=180-1/2∠ABC-90-1/2∠ACB
=90-1/2∠ABC-1/2∠ACB
=1/2α