Probability: Sample Spaces, Mutually Exclusive Events and Sum of Probabilities | 概率：样本空间、互斥事件与概率之和

📚 Probability: Sample Spaces, Mutually Exclusive Events and Sum of Probabilities | 概率：样本空间、互斥事件与概率之和

This revision guide explains the Cambridge KS3 probability topic often tested in Checkpoint. You will learn how to describe likelihood, list all possible outcomes in a sample space, use key probability rules, and avoid common errors. Worked examples and practice questions are included to help you build confidence.

本复习指南讲解剑桥 KS3 概率专题中 Checkpoint 常考内容。你将学习如何描述事件发生的可能性、列出样本空间中的所有可能结果、运用核心概率规则并避免常见错误。文中配有例题和练习题，帮助你建立信心。

1. Probability Scale and Key Terms | 概率标度与关键术语

The probability scale runs from 0 to 1. A probability of 0 means an event is impossible, and a probability of 1 means it is certain. Values such as 1/2 describe events that are equally likely to happen or not happen. You may also see probabilities written as fractions, decimals or percentages, but you should always give your final answer in the form requested by the question.

概率标度从 0 到 1。概率为 0 表示事件不可能发生，概率为 1 表示事件必然发生。像 1/2 这样的值描述发生与不发生可能性相同的事件。概率还可以写成分数、小数或百分数，但最终答案应使用题目要求的形式。

Always check that any probability you calculate is between 0 and 1 inclusive. If you get a negative probability or a probability greater than 1, you have made a mistake.

务必检查你计算出的任何概率是否在 0 到 1 之间（含端点）。如果得出负概率或大于 1 的概率，说明计算中出现了错误。

2. Sample Space Diagrams | 样本空间图

A sample space is the set of all possible outcomes. For a single event, the sample space is simply a list of all outcomes. For two combined events, a sample space diagram is a table showing every possible combination. This helps you count outcomes systematically and avoid missing or double-counting any result.

样本空间是所有可能结果的集合。对于单一事件，样本空间就是所有结果的列表。对于两个组合事件，样本空间图是一张显示所有可能组合的表格，帮助你系统地计数结果

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