A student says that the sequence \(7, 11, 15, 20, \ldots\) is an arithmetic progression because its terms are increasing. What is the error in the student's statement?

Correct answer: The differences between consecutive terms are not equal: \(11-7=4\), \(15-11=4\), but \(20-15=5\).

In an AP, the difference between every pair of consecutive terms must be constant. Here the differences are \(4,4,5\), so it is not an AP. Exam tip: check differences, not merely whether terms increase.