In the figure
AB = AD, BC = CD
prove that ∠ABC = ∠ADC

AB = AD [Given]
BC = CD [Given]
AC = AC [Same line segment]
So, ΔABC ≅ ΔADC [By SSS congruence criteria]
So, ∠ABC = ∠ADC [CPCT (Corresponding parts of congruent triangles)]
Hence, proved.