# 周长 定义

A perimeter is a path that encompasses or surrounds a two-dimensional shape. The term perimeter refers either to the curve constituting the boundary of a lamina or else to the length of this boundary. The term may be used either for the path, or its length in one dimension. It can be thought of as the length of the outline of a shape. For a polygon, the perimeter is the sum of the lengths of the sides. The perimeter of a circle or ellipse is called its circumference although that term is used by some authors to refer to the perimeter of an arbitrary curved geometric figure.

## 周长和面积公式

 几何形状 周长公式 面积公式 符号 长方形 = 2l + 2w A = lw l = 长度，w = 宽度。 正方形 P = 4a A = a2 a = 边长。 三角形 P = a + b + c A = 1⁄2bh a, b, c = 三角形的边长。 b = 三角形的底，h = 三角形的高度。 平行四边形 P = 2b + 2h = bh b = 底，h = 高度。 梯形 P = a + b + c + d A = 1⁄2(a + b)h a, b, c, d = 梯形的边。 a, b = 两个底，h = 垂直高度。 椭圆 P = 2π√ (a2⁄2 + b2⁄2) A = πab a = 长轴半径，b = 短轴半径。 风筝 P = 2a + 2b A = 1⁄2d1d2 a = 第一对等边的长度，b = 第二对等边的长度。 d1 = 风筝长对角线，d2 = 风筝短对角线。 菱形 P = 4a A = 1⁄2d1d2 = 边长。 d1 = 对角线的长度，d2 = 另一个对角线的长度。 五角大楼 = 5a A = 5⁄2sa a = 边长。 s = 五边形的边，a = apothem 长度。 六边形 P = 6a A = 3√ 3 ⁄2a2 a = 边长。 七边形 = 7a A = pa⁄2 a = 边长。 p = 周长值，a = apothem 长度。 八角形 = 8a A = 2a2(1 + √ 2 ) a = 边长。

### 来源

“Perimeter.” Wikipedia, Wikimedia Foundation, 31 Mar. 2020, en.wikipedia.org/wiki/Perimeter.

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