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.
$$\frac{1}{3^2 - 1} + \frac{1}{5^2 - 1} + \frac{1}{7^2 - 1} + \ldots + \frac{1}{(201)^2 - 1}$$ is equal to:
We use partial fractions. Each term has the form $$\frac{1}{n^2 - 1} = \frac{1}{(n-1)(n+1)} = \frac{1}{2}\left(\frac{1}{n-1} - \frac{1}{n+1}\right)$$.
The series is $$\sum_{k=1}^{100} \frac{1}{(2k+1)^2 - 1}$$ where $$n = 2k+1$$ runs through 3, 5, 7, ..., 201. So the sum equals $$\frac{1}{2}\sum_{k=1}^{100}\left(\frac{1}{2k} - \frac{1}{2k+2}\right) = \frac{1}{4}\sum_{k=1}^{100}\left(\frac{1}{k} - \frac{1}{k+1}\right)$$.
This is a telescoping series: $$\frac{1}{4}\left(\frac{1}{1} - \frac{1}{101}\right) = \frac{1}{4} \cdot \frac{100}{101} = \frac{100}{404} = \frac{25}{101}$$.
The answer is Option B: $$\frac{25}{101}$$.
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.