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Question
If nC12 = nC9 find 21Cn
Solution
Given nC12 = nC9
We have nCx = nCy
⇒ x = y or x + y = n
nC12 = nC9
⇒ 12 + 9 = n
⇒ n = 21
⇒ 21Cn = 21C21 = 1
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