Substitution

∠ABM = (1/2) * arc AM and ∠CBM = (1/2) * arc DM

∠CMB = 90

∠AMB = 90

The angles in a linear pair are supplementary

ASA (Angle-Side-Angle)

Inscribed angle measures half its intercepted arc

Circle O with diameter AB, arc AM = arc DM

Inscribed angle measures half its intercepted arc

△AMB ≅ △CMB

Reflexive

arc AB = 180

Given

Substitution

∠ABM = ∠CBM

MB = MB

∠AMB = (1/2) * arc AB

Definition