Solution :
Consider quadrilateral ABCD is a parallelogram. The parallelogram have diagonals AC and DB.
So in the given quadrilateral ABCD, let the diagonal AC and diagonal DB intersects at a point E.
Thus in the quadrilateral ABCD we see that :
1. AC and DB are the diagonals of quadrilateral ABCD.
2. Angle DCE is congruent to angle BAE and angle CDE is congruent to angle ABE. (they are alternate interior angles)
3. Line DC is congruent to line AB. (opposites sides are congruent in a parallelogram )
4. Angle ABE is congruent to angle CDE. (Angle side angle)
5. Line AE is congruent to line EC. And line DE is congruent to line EB. (CPCTC)
Thus we see that if the diagonals of a , then the quadrilateral is a parallelogram.
