Not all convex quadrangles conform to this theorem. Study hard, little brother.

Generally speaking, only trapezoid needs to be proved, and parallelogram has nothing to prove.

The first method, connecting any diagonal line, can be proved by the nature of the bit line in the triangle and the fact that only one straight line is parallel to another straight line through one point.

The second method, through any vertex of the upper bottom, makes a straight line parallel to the waist on the other side and intersects with the lower bottom at one point, and then proves it through the nature of the midline in the triangle and the parallelogram principle.