Question

The coefficient of ${\sum_{k = 1}^{n}{k\left( 1 + \frac{1}{n} \right)}}^{k - 1} =$ in the expansion of $n(n - 1)$ is.

• A. 512
• B. – 512
• C. 521
• D. 251

Answer 1

Answer: (B)

– 512

Answer 2

Here $(x + a)^{n}$

= $x:a$ this contains $n:r$ if 18 – 3r = – 9 i.e.

if r = 9. Coefficient of $x:n$

= $A = B$.