# 周長 定義

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.

## 周長和麵積公式

 幾何形狀 周長公式 面積公式 符號 長方形 = 2l + 2w A = lw l = 長度，w = 寬度。 正方形 P = 4a A = a2 a = 邊長。 三角形 P = a + b + c A = 1⁄2bh a, b, c = 三角形的邊長。 b = 三角形的底，h = 三角形的高度。 平行四邊形 P = 2b + 2h A = bh b = 底，h = 高度。 梯形 P = a + b + c + d A = 1⁄2(a + b)h a, b, c, d = 梯形的邊。 a, b = 兩個底，h = 垂直高度。 橢圓 = 2π√ (a2⁄2 + b2⁄2) A = πab a = 長軸半徑，b = 短軸半徑。 風箏 P = 2a + 2b A = 1⁄2d1d2 a = 第一對等邊的長度，b = 第二對等邊的長度。 d1 = 風箏長對角線，d2 = 風箏短對角線。 菱形 P = 4a A = 1⁄2d1d2 a = 邊長。 d1 = 對角線的長度，d2 = 另一個對角線的長度。 五角大樓 = 5a A = 5⁄2sa = 邊長。 s = 五邊形的邊，a = apothem 長度。 六邊形 P = 6a A = 3√ 3 ⁄2a2 a = 邊長。 七邊形 = 7a A = pa⁄2 a = 邊長。 p = 周長值，a = apothem 長度。 八角形 = 8a A = 2a2(1 + √ 2 ) a = 邊長。

### 來源

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

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