. Find the coefficient of
If | x | < 1, then the coefficient of xn in the expansion of (1 + x + x2 + x3 +…..)2 is
A. n
B. n-1
C. n+2
D. n+1
Best Answer
Ans: D
Sol: We have,
(1 + x + x2 + x3 +…..)2 = [(1 –x)-1]2 = (1 –x)-2
So, coefficient of xn in (1 + x + x2 + x3 +…..)2
= coefficient of xn in (1 –x)-2
= n + 2 –1C2 –1 = n + 1C1 = n + 1
Related questions
