Attempt the question, check the explanation, and continue with similar questions to build confidence before exams.
1 + 2 + 3 + ... + n = n(n + 1)/2
1 + 2 + 3 + ... + n = n^2
1 + 2 + 3 + ... + n = n^3
1 + 2 + 3 + ... + n = n(n - 1)/2
Continue with nearby questions to strengthen the same chapter concept while you still have the context fresh in mind.