Given:
[tex]\text{ BC }\cong\text{ }DA[/tex][tex]AB\text{ }\parallel\text{ }CD\text{ but since BC }\cong\text{ DA, therefore, AB }\cong CD[/tex][tex]\text{ AC is a diagonal line and also the other side that the two triangles share in common.}[/tex]
Because of this, the Triangles are congruent by SSS Theorem.
Side Side Side Postulate (SSS) says triangles are congruent if three sides of one triangle are congruent to the corresponding sides of the other triangle.
