JEE PYQ: Binomial Theorem Question 47
Question 47 - 2019 (10 Jan Shift 1)
If $\sum_{i=1}^{20} \left(\frac{{}^{20}C_{i-1}}{{}^{20}C_i + {}^{20}C_{i-1}}\right)^3 = \frac{k}{21}$, then $k$ equals:
(1) 400 (2) 50 (3) 200 (4) 100
Show Answer
Answer: (4)
Solution
$\frac{{}^{20}C_{i-1}}{{}^{21}C_i} = \frac{i}{21}$. $\sum (i/21)^3 = \frac{(210)^2}{21^3} = \frac{100}{21}$. $k = 100$.