四边形四个内角的和为360°,这是四边形的一个基本性质,这个性质揭示了四边形四个内角之间的关系.(如图1)在凸四边形和凹四边形中,因为周角等于360°,若∠A的外周角(有一个公共顶点和两条公共边并且不重合的两个角,则称其中一个角是另一个角的外周角)为a,则有∠a=∠B+∠C+∠D. The sum of the four internal angles of the quadrilateral is 360°, which is a basic property of the quadrilateral. This property reveals the relationship between the four internal angles of the quadrilateral. (Figure 1) In the convex quadrilateral and the concave quadrilateral, since the angle of the circumference equals 360° If 外A's outer corner (having a common vertex and two common edges and not coincident two corners, then one of the corners is called the outer corner of the other corner) is a, then there is ∠a=∠B+∠C+ D.