一、“中心四边形”的概念及重要命题顺次连接原四边形各边中点得到的新四边形叫“中点四边形”．它的形状受制于原四边形,其重要命题有下列4个：命题1任意四边形的“中点四边形”是平行四边形； 1. The concept of the “central quadrilateral” and important propositions The new quadrangle obtained by sequentially connecting the midpoints of the sides of the original quadrangle is called the “midpoint quadrilateral”. Its shape is subject to the original quadrilateral, and its important propositions have the following four: Proposition 1. The quadrilateral “middle quadrilateral” is a parallelogram;