Class 11 Math: Permutations and Combinations Common Mistakes & Errors

Even students who understand the concepts of Permutations and Combinations often lose marks on exams due to simple, avoidable errors. These "silly mistakes" usually stem from incorrect sign changes, misreading question parameters, or skipping steps. Let's analyze the most common mistakes in this chapter and how to prevent them.

Top Mistakes to Avoid in Exams

Discriminant Sign Errors

Evaluating D = b² - 4ac incorrectly when a or c is negative (e.g., -4 * 1 * -6 should add 24, not subtract 24).

Forgetting the Negative Root

When solving x² = 16, stating only x = 4 and omitting the negative root x = -4.

Dividing by Variables

Simplifying x² = 3x by dividing by x, which loses the valid solution x = 0. Always factorize: x(x - 3) = 0.

How to Eliminate Silly Mistakes

• Highlight Key Numbers: Circle values and units in the question text before writing your answer.
• Check Your Signs Twice: When moving terms across the equals sign, verify that you inverted the sign (e.g. addition to subtraction).
• Estimate First: Work out an approximate answer mentally. If your detailed calculation is completely different, you likely made a basic decimal error.