Parallelogram: Definition: Two groups of parallelograms with opposite sides are parallelograms.

Properties: parallelogram has: ① two sides are parallel and equal,

② diagonal lines are equal.

③ diagonal bisection,

Five judging methods: ① Two groups of quadrangles with equal opposite sides (length) are parallelograms.

② A set of quadrilaterals with parallel and equal opposite sides is a parallelogram.

③ A quadrilateral whose diagonal is bisected is a parallelogram.

④ Two groups of quadrangles with equal diagonal angles are parallelograms.

⑤ Two groups of quadrangles with equal opposite sides are parallelograms.