Top 10 Mistakes in A Level Mathematics — And How to Stop Making Them

Most A Level Mathematics students lose 12–20 marks across their papers to mistakes that have nothing to do with topic knowledge. Fix these, and you can lift a B to an A — or an A to an A* — without learning a single new concept. Here are the ten mistakes I see most often, with the exact fix for each.

1. Calculator left in degrees during a radians question

The single most expensive mistake on Pure papers. A trigonometric integral evaluated in degrees gives a wildly wrong answer. The fix: check the calculator mode before every trig or calculus question. Make it a habit, not a hope.

2. Forgetting "+C" on indefinite integrals

A one-mark loss every time it happens — and it happens on 40% of integration questions in mock papers I mark. The fix: write "+C" before you do the integration, then fill in the rest.

3. Dropping the modulus sign

When solving |2x – 3| = 5, students solve 2x – 3 = 5 and stop. They miss 2x – 3 = –5 and lose half the marks. The fix: every time you see a modulus, write two equations immediately.

4. Quadratic formula with the wrong sign on b

b² – 4ac, not (–b)² – 4ac interpreted as b² – 4ac only when b is positive. If b = –5, then (–5)² = 25. The fix: write a, b, c separately at the top before substituting.

5. Confusing dy/dx = 0 with the endpoint maximum

Stationary points are not always the maximum on a closed interval. The fix: always check the function value at the endpoints, not just at the stationary points.

6. Resolving forces along the wrong axes in Mechanics

On an inclined plane, students resolve along horizontal and vertical axes instead of along and perpendicular to the slope. The maths works but takes three times longer and introduces sign errors. The fix: rotate your axes to match the geometry of the problem.

7. Misreading "exactly" in trigonometry

"Find the exact value of sin(75°)" requires a surd answer, not a decimal. A decimal answer scores zero. The fix: underline "exact", "in terms of π", and "in surd form" every time they appear.

8. Using degrees of freedom incorrectly in Statistics

For a one-sample t-test with n = 12, degrees of freedom is n – 1 = 11, not n. The fix: write df = ? next to the test statistic and confirm the value before looking up the critical region.

9. Forgetting to differentiate implicitly

When the curve is defined as x²y + sin(y) = 4, students differentiate the left side as if y were a constant. The fix: every time you see y inside a function of x, write dy/dx next to it as you differentiate. Build the chain rule into the muscle memory.

10. Running out of time on the last question

Most A Level Maths papers have a tough multi-part question at the end. Students who pace badly lose 12–15 marks not because they cannot do it, but because they never reach it. The fix: in every mock, glance at the clock after each question. If you fall behind, skip and return. The last question is usually the most generous on method marks.

The bonus mistake: not reviewing your own work

Students who never re-mark their own past papers repeat the same mistakes for months. The fix: after every full paper, classify every lost mark into one of these ten categories — and target next week's practice at the most common one.

How to embed the fixes

Reading this list once does not embed the fixes. Print it out. Pin it above your desk. Review it before every practice session for two weeks. After two weeks, the fixes become reflexes — and those reflexes are what an A* is made of.

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