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  4. If 21 C 1+5 ⋅ 21 C 5+9 ⋅ 21 C 9+ ldots .+17 ⋅ 21 C 17+21 ⋅ 21 C 21= k. Then number of prime factors of k is

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Q. If ${ }^{21} C _1+5 \cdot{ }^{21} C _5+9 \cdot{ }^{21} C _9+\ldots .+17 \cdot{ }^{21} C _{17}+21 \cdot{ }^{21} C _{21}= k$. Then number of prime factors of $k$ is

Binomial Theorem

Solution:

Correct answer is (c) 4