find n if 3.n+1 c 3=7×nc2
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3 (n+1)C3 = 7 nC2
3 (n+1)! / ((n-2)! 3!) = 7 (n! / ((n-2)! 2!)
3(n-1)n! / 3! = 7 n! / 2!
3 (n-1) = 7*6/2
3n = 21 + 3
n = 8
3 (n+1)! / ((n-2)! 3!) = 7 (n! / ((n-2)! 2!)
3(n-1)n! / 3! = 7 n! / 2!
3 (n-1) = 7*6/2
3n = 21 + 3
n = 8
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