0 votes 0 votes Let $S=2x + 5x^2 + 9x^3 + 14x^4 + 20x^5 \dots \dots $ infinity. The coefficient of $n$-th term is$\frac{n(n+3)}{2}$. Then the sum $S$ is $\frac{x(2-x)}{(1-x)^3}$ $\frac{(2-x)}{(1-x)^3}$ $\frac{x(2-x)}{(1-x)^2}$ None of these Quantitative Aptitude cat2002 quantitative-aptitude infinite-geometric-progression + – go_editor asked Mar 2, 2016 • edited Apr 9, 2022 by Lakshman Bhaiya go_editor 13.9k points 247 views answer comment Share See all 0 reply Please log in or register to add a comment.