Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
The sum of the infinite series $$1 + \frac{5}{6} + \frac{12}{6^2} + \frac{22}{6^3} + \frac{35}{6^4} + \frac{51}{6^5} + \frac{70}{6^6} + \ldots$$ is equal to:
We need to find the sum: $$S = 1 + \frac{5}{6} + \frac{12}{6^2} + \frac{22}{6^3} + \frac{35}{6^4} + \frac{51}{6^5} + \frac{70}{6^6} + \ldots$$ The numerators are 1, 5, 12, 22, 35, 51, 70, ... whose first differences 4, 7, 10, 13, 16, 19, ... form an arithmetic progression with common difference 3, and whose constant second differences 3 imply a quadratic general term: $$a_n = An^2 + Bn + C$$.
Using $$a_1 = 1, a_2 = 5, a_3 = 12$$ leads to the system $$A + B + C = 1,\quad 4A + 2B + C = 5,\quad 9A + 3B + C = 12$$, which simplifies to $$3A + B = 4,\quad 5A + B = 7$$ and yields $$A = \frac{3}{2},\; B = -\frac{1}{2},\; C = 0$$. Thus $$a_n = \frac{3n^2 - n}{2} = \frac{n(3n-1)}{2}\;.$$
Writing the series with $$r = \frac{1}{6}$$ gives $$S = \sum_{n=1}^{\infty} \frac{n(3n-1)}{2} \left(\frac{1}{6}\right)^{n-1} = \frac{3}{2}\sum_{n=1}^{\infty}n^2 r^{n-1} - \frac{1}{2}\sum_{n=1}^{\infty}n\,r^{n-1}$$ where $$r = \frac{1}{6}$$.
Using the standard formulas $$\sum_{n=1}^{\infty} n r^{n-1} = \frac{1}{(1-r)^2} = \frac{1}{(5/6)^2} = \frac{36}{25}$$ and $$\sum_{n=1}^{\infty} n^2 r^{n-1} = \frac{1+r}{(1-r)^3} = \frac{1+1/6}{(5/6)^3} = \frac{7/6}{125/216} = \frac{7}{6} \times \frac{216}{125} = \frac{252}{125}\;,$$ we obtain $$S = \frac{3}{2}\times\frac{252}{125} - \frac{1}{2}\times\frac{36}{25} = \frac{756}{250} - \frac{36}{50} = \frac{756}{250} - \frac{180}{250} = \frac{576}{250} = \frac{288}{125}\;.$$
Therefore, the answer is Option C: $$\boldsymbol{\frac{288}{125}}$$.
Create a FREE account and get:
Predict your JEE Main percentile, rank & performance in seconds
Educational materials for JEE preparation
Ask our AI anything
AI can make mistakes. Please verify important information.
AI can make mistakes. Please verify important information.