List of top Quantitative Aptitude Questions on Series

Find the missing number in the series: \( 2, 6, 12, 20, 30, \_\_\_\_ \).

If \( x = 1 + 2a + 3a^2 + 4a^3 + \ldots \) (\(|a| < 1\)) and \( y = 1 + 3b + 6b^2 + \ldots\) (\(|b| < 1\)), then find \( 1 + ab + (ab)^2 + (ab)^3 + \ldots \) in terms of \(x\) and \(y\).