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Home  >>  CBSE XI  >>  Math  >>  Permutations and Combinations
+1 vote

(n+2)! = 60(n-1)!

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1 Answer

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Given $ (n+2)! = 60 (n-1)!$
$\Rightarrow (n+2)! = (n+2) (n+1) n!$
Hint: Now, $n! = n (n-1)!$
$\Rightarrow (n+2)! = (n+2) (n+1) n (n-1)!$
The equation now resolves to $(n+2) (n+1) n (n-1)! = 60 (n-1)!$
Canceling out $(n-1)$ on both sides, $(n+2) (n+1) n = 60$
$\Rightarrow (n^2 + 2n + n + 2)n = 60$
$\Rightarrow n^3 + 3n^2 + 2n = 60$
$\Rightarrow n^3 + 3n^2 + 2n - 60 = 0$
Factoring the left hand side, this equation resolves to $(n-3) (n^2+6n+20) = 0$
Since it can be shown that $(n^2 + 6n + 20) = 0$ has no real solution, $n = 3$
Solution: $n = 3$

 

answered Jun 4, 2013 by balaji.thirumalai
 

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