A) 1085
B) 560
C) 680
D) 1240
Correct Answer: C
Solution :
\[\sum\limits_{r=1}^{15}{{{r}^{2}}}\left( \frac{^{15}{{C}_{r}}}{^{15}{{C}_{r-1}}} \right)=\sum\limits_{r=1}^{15}{{{r}^{2}}}\left( \frac{15-r+1}{r} \right)=\sum\limits_{r=1}^{15}{r}(16-r)=16\] \[\left( \frac{15\times 16}{2} \right)-\frac{15\times 16\times 31}{6}=\frac{15\times 16}{6}(17)=680.\]
You need to login to perform this action.
You will be redirected in
3 sec
