If (1+x)^n=C_0+C_1x+C_2x^2+ +C_nx^n, then _ 0 r<s n (r+s)C_rC_s is equal to

JEE Main Mathematics question

If , then is equal to

Answer: (A)

Over all ordered pairs, , and the diagonal terms give . The pair sum is half of the off-diagonal total:

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Question ID VK-021139