How we check answers.

A wrong answer that you memorise is worse than no answer at all. That is why every question goes through these steps before you see it.

1. The source is kept. We record where a question came from, such as the book and page, together with the answer printed there.
2. The answer is solved again. Each question is solved again independently — by an editor, or by an AI model that also writes the short explanation. When this check disagrees with the printed answer, or is not sure, the question is flagged instead of being copied.
3. An editor decides anything doubtful. A person reviews every flagged question and corrects it when needed. Only questions where the independent check is certain (and agrees with the book when the book gives an answer) are published without waiting for an editor.
4. You can still report it. If you spot a problem, use “Report a mistake” on the question. The editors review every report.

What the labels mean

• Checked by an editor — a person has approved the question and its answer.
• AI-checked — the answer was solved independently by AI with high confidence; editors look at it as soon as anyone reports a problem.
• Corrected — the answer printed in the source was wrong and has been fixed.

Questions and exams change over time. If something looks outdated, please tell us.