1. Two groups of parallelograms with parallel opposite sides are parallelograms (definition judgment method);

Determination of parallelogram

Determination of parallelogram

2. A group of quadrilaterals with parallel and equal opposite sides are parallelograms;

3. The quadrilateral whose diagonals bisect each other is a parallelogram;

4. Two groups of quadrangles with equal diagonal are parallelograms; (Example 3)

5. The quadrilateral with complementary adjacent angles (each group of adjacent angles) is a parallelogram;

6. Two sets of quadrilaterals with equal opposite sides are parallelograms.

(1): the opposite sides of the parallelogram are equal respectively;

(2): The opposite sides of the parallelogram are parallel respectively;

(3): The diagonals of parallelograms are equal respectively;

(4): diagonal bisection of parallelogram;

(5): The adjacent angles of the parallelogram are complementary.

It's natural.

Judgment is the inverse proposition of nature.