The Most Common Mistakes in IB Mathematics — And How to Avoid Them

This is the single most costly mistake in IB Mathematics, and it happens more than you'd think. A student uses their calculator, gets the right answer, writes it down — and earns one mark out of four. The method marks, which make up the majority of the marks in most questions, require working to be shown. A correct answer alone is not enough.

Write down every step, even the ones that feel obvious. An examiner only awards marks for work that they can see. Write the setup, the method, and the key intermediate steps before writing the final answer.

This mistake is subtle and surprisingly common. A student rounds an intermediate value — say, keeping three significant figures instead of the full calculator value — and carries that rounded number into the next step. By the time they reach the final answer, it falls just outside the accepted range in the mark scheme. The answer looks right. It might even feel right. But it earns no accuracy mark.

Keep full calculator values throughout your working. Only round at the very last step, to the degree of accuracy specified in the question. If no accuracy is specified, three significant figures is the IB standard.

Every IB Math question contains a command term — write down, find, show that, hence, justify, prove. These are not decorative. They tell you exactly what the examiner expects and how they will mark your response.

"Write down" means no working is required — but also that no marks are available for method, so an incorrect answer earns nothing. "Hence" means you must use the result from the previous part. Any other method, however correct, will not earn method marks. "Justify" means a correct answer without a reason is incomplete.

Before attempting any question, read the command term first. Let it tell you what kind of response is expected before you start writing.

This happens most often in questions involving hypothesis testing, optimisation, or proof. A student completes all the correct calculations, arrives at the right value, and stops. No concluding statement. The last mark — which is almost always an R mark for reasoning — is not awarded. The mathematics is there, but the answer is incomplete.

Always finish your answer with a statement that directly addresses the question. If the question asks whether a function has a minimum, don't just find the second derivative — state explicitly what it tells you and why. If the question asks you to determine whether a result is significant, state your conclusion clearly in context.

Under exam pressure, students sometimes cross out working they are unsure about, and replace it with something incorrect — or nothing at all. Once working is crossed out, an examiner cannot mark it. If that crossed-out working was correct, those marks are gone.

If you want to redo a solution, start it fresh on a new section of the page and label it clearly. Leave the original working intact unless you are completely certain it is wrong. For Section A, it is perfectly acceptable to write outside the given space — use additional booklets if needed, clearly labelled. An examiner will mark the clearest, most complete attempt — but only if it's visible.

"Show that" questions behave differently from every other question type. The answer is given to you in the question. The examiner is not marking your answer — they are marking your path to it. Every logical step must be shown explicitly. Assumptions cannot be skipped. Circular reasoning — using the result to prove the result — earns no marks.

Many students rush "show that" questions because the answer feels obvious. That's exactly when marks are lost.

Slow down on "show that" questions. Write every step as if the person reading your work has never seen the question before. Your working must arrive at the given answer through clear, unambiguous steps — not assume it.

This one is specific to HL students. Paper 3 is 60 minutes, two long investigations, and can be as long as 30 marks per question. The most common time management mistake is spending too long on a part you find difficult, losing the time needed to attempt the parts that follow.

If you are stuck on a part, write what you can, move on, and come back if time allows. In P3, the later parts of an investigation are not always harder than the earlier ones. Keep moving. For a full guide on how to approach P3, read our Paper 3 article.

This sounds basic. It costs marks at every level, every year. Students misread the domain of a function, miss the word "exact," overlook "in the form a + bi," or answer for the wrong variable. The mathematics is correct — but it answers a question that wasn't asked.

After completing each answer, re-read the original question. Check that what you've written actually answers what was asked — the right variable, the right form, the right degree of accuracy.