FORM 2 MATHS [theory]

對於polygon, 唔同人(或者書)會有唔同既定義,
in Wikipedia 就有兩個唔同既定義:
(1)
In geometry a polygon (IPA: [ˈpɒliɡən], from the Classical Greek πολυγον, meaning literally "many-knee" or "many-angle") is a plane figure that is bounded by a closed path or circuit, composed of a finite number of sequential line segments. The straight line segments that make up the boundary of the polygon are called its edges or sides and the points where the edges meet are the polygon's vertices or corners. The interior of the polygon is its body.

(2)
Polygons are named and classified according to the number of sides, combining a Greek-derived numerical prefix with the suffix -gon, e.g. pentagon, dodecagon. The triangle, quadrilateral, and nonagon are exceptions. For large numbers, mathematicians usually write the numeral itself, e.g. 17-gon. A variable can even be used, usually n-gon. This is useful if the number of sides is used in a formula.

如果根據(1), 四邊形係多邊形既一種, 所以係可以用∠sum of polygon
而(n-2)÷180 條式係完全正確的
∠sum=(4-2)÷180=360 (∠sum of polygon)
[n 係代表有幾多條邊, 即係4]
原因就如原因就如de_familiar_stranger 講咁你可以cut 開個四邊形做兩個三角形

但如果你本書(或者你老師)係覺得四邊形唔係多邊形的話
我地多數就會好似leuk115 咁
用(∠ sum of quad.)

一定係360°
(∠ sum of quad.)

2007-02-23 13:38:02 補充：
條theory係 (n-2)x180

2007-02-23 14:09:06 補充：
我唔明why ∠sum=(4-2)÷180=360 (∠sum of polygon)