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Q.
The value of $\frac{1}{1 ! 50 !}+\frac{1}{3 ! 48 !}+\frac{1}{5 ! 46 !}+\ldots .+\frac{1}{49 ! 2 !}+\frac{1}{51 ! 1 !}$ is :
JEE MainJEE Main 2023Permutations and Combinations
Solution:
$ \displaystyle\sum_{ r =1}^{26} \frac{1}{(2 r -1) !(51-(2 r -1)) !}=\displaystyle\sum_{ r =1}^{26}{ }^{51} C _{(2 r -1)} \frac{1}{51 !} $
$ =\frac{1}{51 !}\left\{{ }^{51} C _1+{ }^{51} C _3+\ldots .+{ }^{51} C _{51}\right\}=\frac{1}{51 !}\left(2^{50}\right)$