Connect OC, OC', OB'
According to the symmetry ∠ BOC = ∠ b 'oc' ∠ com = ∠ c 'om ∠ b'' OE = ∠ b 'oe.
∠BOB ' ' =∠BOC+∠COM+∠C ' om+∠B ' oc '+∠B ' ' OE+∠B ' OE
= 2(≈C ' om+≈B ' oc '+≈B ' OE)
=2∠MOE
=2α