A plane figure with four sides and four angles — a quadrilateral (also called a quadrangle).
正方形はすべての辺が等しい特別な四辺形だ。
A square is a special quadrilateral whose all sides are equal.
問題文にある四辺形ABCDの面積を求めなさい。
Find the area of quadrilateral ABCD given in the problem statement.
三角形と四辺形の性質の違いを説明してください。
Please explain the differences in properties between triangles and quadrilaterals.