Answer variants:
The sides of a parallelogram bisect each other.
The line segment joining the mid-points of two sides of a triangle is perpendicular to the third side.
If each pair of opposite sides of a quadrilateral is not equal, then it is a parallelogram.
A diagonal of a parallelogram divides it into two congruent triangles.
If the diagonals of a quadrilateral bisect each other, then it is a parallelogram.
A quadrilateral is a parallelogram if a pair of opposite sides is equal and parallel.
Choose any three correct theorems.
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3.