Question

The sum of the first three coefficients in the expansion `(x+(1)/(y))^(n)` is 22. Find the value of n.
A. 8
B. 7
C. 6
D. 5

Answer 1

Correct Answer - C
Given that `""^(n)C_(0)+""^(n)C_(1)+""^(n)C_(2)=22`
`rArr1+n+(n(n-1))/(2)=22`
`rArr+2+2n+n^(2)-n=44`
`rArr n^(2)+n-42=0`
`rArr(n+7)(n-6)=0`
`rArr n=-7 or n=6`