A-Level Edexcel Maths: Common Mistake Questions Explained | A-Level Edexcel 数学：易错题精讲

📚 A-Level Edexcel Maths: Common Mistake Questions Explained | A-Level Edexcel 数学：易错题精讲

Many A-Level Edexcel Maths students lose marks not because they don’t understand the concepts, but because they fall into common traps. This article dissects ten frequently seen mistake patterns and shows how to avoid them with clear, step-by-step corrections.

许多 A-Level Edexcel 数学学生丢分并非因为不理解概念，而是因为掉进了常见的陷阱。本文剖析了十个最常见的错误模式，并通过清晰的逐步纠正来展示如何避免这些错误。

1. Expanding Brackets and Sign Errors | 展开括号与符号错误

A classic error is treating (x − 3)² as x² − 9, forgetting the middle term. Correct expansion: (x − 3)² = x² − 2·x·3 + 3² = x² − 6x + 9.

经典错误是把 (x − 3)² 当成 x² − 9，忘记了中间项。正确展开：(x − 3)² = x² − 2·x·3 + 3² = x² − 6x + 9。

Similarly, when expanding (2x + 1)(x − 4), some students write 2x² − 7x − 4 incorrectly. The correct steps: First: 2x·x = 2x²; Outer: 2x·(−4) = −8x; Inner: 1·x = x; Last: 1·(−4) = −4. Summing: 2x² − 7x − 4. The mistake often occurs in signs with −8x + x = −7x, but some get −9x.

同样，在展开 (2x + 1)(x − 4) 时，有些学生错误地写成 2x² − 7x − 4。正确步骤：首项：2x·x = 2x²；外项：2x·(−4) = −8x；内项：1·x = x；末项：1·(−4) = −4。合并得到 2x² − 7x − 4。错误常出现在符号上，−8x + x = −7x，但有人算成 −9x。

When expanding (a − b)³, a common slip is missing alternating signs: correct is a³ − 3a²b + 3ab² − b³. Writing all plus signs is wrong.

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