0 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 14.2k points 964 views answer comment Share Follow Print 0 reply Please log in or register to add a comment.