概率之和等于1

理解当一次试验的所有可能结果都列出来时，它们的概率相加必须等于1；利用这一点可以求出某事件不发生的概率：P(非A) = 1 − P(A)；运用这个简便方法，避免直接数出所有不利结果。

Understand that when all possible outcomes of a trial are listed, their probabilities must add up to 1; use this to find the probability of an event NOT happening: P(not A) = 1 − P(A); apply this shortcut to avoid counting all unfavourable outcomes directly

学习证据 / Evidence

• 列出转动一个四色转盘的所有可能结果，并验证它们的概率相加等于1。
• 利用对立事件规则，计算出不掷出3的概率为1 − 1/6 = 5/6。
• 找出一张概率表中数值加起来不等于1的错误，并解释错在哪里。

• List all outcomes of spinning a 4-colour spinner and verify their probabilities add up to 1
• Calculate P(not rolling a 3) as 1 − 1/6 = 5/6 using the complement rule
• Spot an error in a probability table where the values don't sum to 1 and explain what's wrong

评估问题 / Assessment

如果明天下雨的概率是0.3，{{name}}能算出明天不下雨的概率，并说明为什么同一情境中所有概率相加都得1吗？

If the probability of it raining tomorrow is 0.3, can {{name}} work out the probability of it NOT raining — and explain why all probabilities in a situation must add up to 1?