“燕尾”四边形就是常见的凹四边形,因它的形状象剪刀,因此把它称为“燕尾”四边形.我们以前学习和研究的四边形都是凸四边形,其实凹四边形也有很多的性质值得我们去研究. The “swallowtail” quadrangle is a common concave quadrilateral. Because it is shaped like a scissors, it is called a “swallowtail” quadrangle. The quadrilaterals we studied and studied previously are all convex quadrilaterals. In fact, concave quadrilaterals have many properties. It is worth our while to study.