Common Mistakes and How to Avoid Them

This lesson catalogues the most frequent errors that Edexcel A-Level Mathematics students make, drawn from examiner reports across multiple years of the 9MA0 specification. For each mistake, we explain why it happens and how to avoid it. Eliminating these errors is one of the fastest ways to improve your grade.

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Algebraic Errors

Algebra underpins every topic in A-Level Mathematics. Small algebraic mistakes cascade through multi-step questions and cost multiple marks.

Mistake 1: Sign Errors

The problem: Students drop or change signs when expanding brackets, rearranging equations, or working with negative numbers.

Examples:

• Writing -(3x - 2) = -3x - 2 instead of -3x + 2
• Writing (-2)² = -4 instead of 4
• Losing the negative when moving a term across an equation

How to avoid it:

• When expanding a negative bracket, write out every term individually before simplifying
• Circle or underline negative signs as you write them
• After rearranging, substitute your answer back into the original equation to verify

Mistake 2: Incorrect Index Laws

The problem: Students misapply laws of indices, particularly with fractional and negative powers.