![\"\"](\"/userfiles/image/steps_images/solution.JPG\")
Since one of the interior angles form linear pair with corresponding external angle,
\its angle measure is ![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=7270e10404238ece3d526da250c0e15c.png\")
.
Each pair of opposite angles has the same measure. So, they are congruent.
\If both pairs of opposite angles are congruent, then the quadrilateral is a parallelogram.