prove that a quadrilateral with both pairs of opposite sides congruent must be a parallelogram given segment PL is congruent to segment GM AND SEGMENT LG is congruent to segment MP proves PLGM is a parallelogram
see the attached figure
we have that
1) PL=GM -------> given
2) LG=MP-------> given
3) ML=LM -----> Common side, reflexive property of equality
4) ΔMPL ≅ΔLGM -------> SSS ;postulate
5) ∠MPL≅∠LGM ------> by Corresponding angles in congruent triangles
that prove that the quadrilateral has opposite sides parallel and congruent, and opposite angles congruent, that is the definition of the parallelogram