但是对于一个四边形,已知其四条边的长度口a,b,c,d,也不一定能算出其面积。如已知菱形的边长为a,就算不出此菱形的面积,这是为什么呢:这是因为三角形有稳定性,而四边形不一定有稳定性。或者说是因为任何三角形都有外接圆,而四边形就不一定有外接圆,因此有无外接圆,可把 However, for a quadrilateral, the lengths a, b, c, d of its four sides are not necessarily calculated. If the side length of a diamond is known as a, if the area of ​​the diamond is not available, why? This is because the triangle has stability and the quadrilateral does not necessarily have stability. Or because any triangle has a circumscribed circle, and the quadrilateral does not necessarily have a circumscribed circle, so whether or not there is a circumscribed circle can be