Step-by-step explanation:
cd and ad are chords
in triangles aeb and ced
angle aed = angle ced (vertically opposite angles)
angles made by 2 points on the circumference to one single point on the circle is inscribed angles
in a circle,inscribed angles are equal
say chord cb makes the inscribed angle cab and angle bdc
these 2 angles are equal, inscribed angles theorem
same way chord ad makes angle DBA and angle acd which are equal by inscribed angles theorem
so 3 angles in each triangle are equal respectively.
by AAA theorem, the triangles are similar
