📚 Angle Properties and Parallel Lines | 角度性质与平行线
In this revision guide, we cover the IGCSE Mathematics topic G-2, focusing on angle properties and parallel lines. You will learn the essential definitions, rules, and problem-solving techniques needed to tackle common exam questions confidently.
在本复习指南中，我们将学习 IGCSE 数学 G-2 板块：角度性质与平行线。你将掌握关键定义、规则和解题技巧，从而自信地应对常见考题。
1. Basic Angle Types | 基本角度类型
Angles are measured in degrees (°). Understanding the basic types is essential for recognizing relationships in diagrams. An acute angle is less than 90°, a right angle is exactly 90°, and an obtuse angle is between 90° and 180°.
角度以度（°）为单位。理解基本类型对于识别图形中的关系至关重要。锐角小于 90°，直角恰好为 90°，钝角在 90° 与 180° 之间。
A straight angle is exactly 180°.
平角恰好为 180°。
A reflex angle is between 180° and 360°.
优角（反角）在 180° 与 360° 之间。
A full turn is 360°.
一周角为 360°。
| Type | Measure | 类型 | 度数 |
| Acute | 0° - 90° | 锐角 | 0° - 90° |
| Right | 90° | 直角 | 90° |
| Obtuse | 90° - 180° | 钝角 | 90° - 180° |
| Straight | 180° | 平角 | 180° |
| Reflex | 180° - 360° | 优角 | 180° - 360° |
2. Angles on a Line and at a Point | 直线上的角与周角
On a straight line, the sum of adjacent angles is always 180°. This is known as the "angle sum on a straight line". At a point, the sum of all angles around a point is always 360°.
在一条直线上，相邻角度之和恒为 180°，这称为“直线上角的和”。一个点周围所有角度之和恒为 360°。
a + b = 180° (straight line) 且 a + b + c + d = 360° (point)
When two lines intersect, vertically opposite angles are equal. For example, if two lines cross, the angles facing each other are the same.
当两条直线相交时，对顶角相等。例如，两线交叉时，相对的两个角度相同。
3. Parallel Lines and Transversals | 平形线与截线
A transversal is a line that crosses two parallel lines. This creates several angle relationships: corresponding angles, alternate angles, and co-interior angles.
截线是穿过两条平行线的直线。这会形成多种角度关系：同位角、内错角与同旁内角。
Corresponding angles are equal (F pattern).
同位角相等（F 型）。
Alternate angles are equal (Z pattern).
内错角相等（Z 型）。
Co-interior angles sum to 180° (C pattern).
同旁内角之和为 180°（C 型）。
Corresponding: ∠a = ∠b
Alternate: ∠c = ∠d
Co-interior: ∠e + ∠f = 180°
These patterns help you find missing angles quickly. Always look for the F, Z, and C shapes in the diagram.
这些图形能帮助你快速找到缺失角度。在图中识别 F、Z、C 形状即可。
4. Angles in Triangles | 三角形内角
The interior angles of a triangle always add up to 180°. This is a fundamental property used in many geometry problems.
三角形内角和恒为 180°，这是许多几何问题中用到的基本性质。
x + y + z = 180°
The exterior angle of a triangle is equal to the sum of the two opposite interior angles. For a triangle with angles A, B, and C, the exterior angle at vertex C equals A + B.
三角形的外角等于与它不相邻的两个内角之和。对于三角形，顶点 C 处的外角等于 A + B。
Special triangles:
Equilateral triangle: all angles are 60°.
等边三角形：每个角均为 60°。
Isosceles triangle: two sides equal, two base angles equal.
等腰三角形：两边相等，两个底角相等。
Right-angled triangle: one angle is 90°.
直角三角形：一个角为 90°。
5. Angles in Quadrilaterals | 四边形内角
The sum of interior angles in any quadrilateral is 360°. This follows from dividing the quadrilateral into two triangles.
任何四边形内角和为 360°，这是通过将四边形分成两个三角形得出的。
w + x + y + z = 360°
Special quadrilaterals have unique angle properties:
Square and rectangle: all angles are 90°.
正方形和长方形：所有角均为 90°。
Parallelogram: opposite angles are equal, adjacent angles sum to 180°.
平行四边形：对角相等，邻角之和为 180°。
Trapezium (trapezoid): sum of adjacent angles on each leg is 180°.
梯形：每一条腰上的邻角之和为 180°。
6. Interior and Exterior Angles of Polygons | 多边形内角与外角
la: (n - 2) × 180°. For example, a hexagon (n = 6) has interior sum (6 - 2) × 180° = 720°.
对于任何 n 边形，内角和公式为： (n - 2) × 180°。例如，六边形（n = 6）的内角和为 (6 - 2) × 180° = 720°。
Sum of interior angles = (n - 2) × 180°
r polygon, each exterior angle is 360° / n, and each interior angle is (n - 2) × 180° / n.
任何多边形的外角和恒为 360°，与边数无关。对于正多边形，每个外角为 360° / n，每个内角为 (n - 2) × 180° / n。
Exterior angle (regular) = 360° ÷ n
Interior angle (regular) = (n - 2) × 180° ÷ n
7. Angle Problems with Algebra | 结合代数的角度问题
Many IGCSE questions require setting up equations using angle properties. For example, if two angles are named as expressions like (2x + 10)° and (3x - 20)°, and they are alternate angles, you can equate them to find x.
许多 IGCSE 题目要求利用角度性质列出方程。例如，若两个角表示为 (2x + 10)° 和 (3x - 20)°，且它们为内错角，则可令它们相等求出 x。
Worked Example:
Find x if the angles 2x + 30° and 3x - 10° are corresponding angles (equal).
例题：
若角 2x + 30° 和 3x - 10° 为同位角（相等），求 x。
2x + 30 = 3x - 10 → x = 40
Always state the angle rule you are using, e.g., "corresponding angles are equal". This helps earn method marks.
解题时务必写出所用角度规则，例如“同位角相等”，这样能获得方法分。
8. Common Exam Tips and Worked Problems | 常见考试技巧与典型题
In this final section, we share practical tips that are specifically useful for IGCSE exams.
在最后一部分，我们分享对 IGCSE 考试特别实用的技巧。
Always annotate the diagram with known angles to avoid confusion.
务必在图中标出已知角度，避免混淆。
Use a protractor only if the question asks for measurement; otherwise, calculate using rules.
只有题目要求测量时才用量角器；否则使用规则计算。
Be careful with parallel line patterns when lines are not horizontal.
当平行线不是水平时，要格外注意平行线的角度模式。
For polygon questions, determine if the polygon is regular or irregular; regular polygons have all equal angles.
对于多边形题目，判断是正多边形还是任意多边形；正多边形的所有角相等。
Full Worked Example:
In the diagram below, two parallel lines are cut by a transversal. One angle is 55°. Find the measure of the alternate angle and the co-interior angle.
完整例题：
在图中，两条平行线被一条截线所截，其中一个角为 55°。求内错角和同旁内角的度数。
Alternate angle = 55°
Co-interior angle = 180° - 55° = 125°
Understanding the angle rules and practising with algebraic expressions will help you solve a wide range of problems quickly and accurately.
理解角度规则并熟练结合代数表达式练习，能帮助你快速准确地解决各类问题。
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