Q. The sum of the coefficients in the expansion of $\left( 1 + 2x + 3x^{2} + ... + nx^{n} \right)^{2}$ is
A) $\sum_{}^{}1$
B) $\sum_{}^{}n$
C) $\sum_{}^{}n^{2}$
D) $\sum_{}^{}n^{3}$
Explanation
(d)
Put $x = 1$, we get
${(1 + 2 + 3 + \ldots + n)}^{2} = \sum_{}^{}n^{3}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.