a plane figure的用法和样例：
- A plane figure with four sides and four angles.
- A plane figure bounded by two radii and the included arc of a circle.
- An instrument that measures the area of a plane figure as a mechanically coupled pointer traverses the perimeter of the figure.
- A plane figure consisting of four points,no three of which are collinear,connected by straight lines.
- A four-sided plane figure with four right angles.