If angle ABC and angle ADC are equal then the triangle ABC and triangle ADC is congruent by SAS.
Both the triangles have one side in common that is AC.
The length of the base of the triangle is the same, i.e. BC=CD.
In order to prove the triangles congruent using the SAS congruence postulate, we need the other information, likewise any of the angles that are equivalent in both the triangles. If angle ABC and angle ADC are equal then the triangle ABC and triangle ADC is congruent by SAS.
We required two sides and one angle equivalent in two triangles to prove these triangles are congruent by SAS axiom.
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