Common Exam Mistakes

Knowing the common mistakes that cost students marks on Edexcel GCSE Mathematics — and how to avoid them — is one of the fastest ways to improve your grade. This lesson catalogues the most frequent errors and gives you strategies to prevent each one.

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Mistake 1: Sign Errors

Sign errors (getting positives and negatives mixed up) are the single most common source of lost marks.

Where Sign Errors Happen

• Expanding brackets with negatives: −3(x − 4) = −3x + 12 (NOT −3x − 12)
• Collecting like terms: 5x − 3 − 2x + 7 = 3x + 4 (watch the signs on every term)
• Substituting negative values: If x = −2, then x² = (−2)² = 4, NOT −4
• Solving equations: When you move a term across the equals sign, the sign changes
• Negative coordinates: Misplotting (−3, 2) as (3, −2)

How to Avoid Sign Errors