π \pi (...) something!

Algebra Level 5

If π ( n ) \pi(n) denotes product of all binomial coefficients in ( 1 + x ) n \left(1+ x\right)^n , then the ratio of π ( 2002 ) \pi(2002) to π ( 2001 ) \pi(2001) can be expressed as

200 2 m n ! , \dfrac{2002^{m}}{n!},

where m m and n n are positive integers. Fin the minimum value of m + n m+n .


Image Credit: Wikimedia Pascal Triangle by Dohduhdah


The answer is 4002.

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2 solutions

Jake Lai
Mar 15, 2015

By the binomial theorem,

π ( n ) = ( n 0 ) ( n 1 ) ( n n ) = n ! n + 1 0 ! 2 1 ! 2 n ! 2 \pi(n) = {n \choose 0} {n \choose 1} \ldots {n \choose n} = \frac{n!^{n+1}}{0!^{2} 1!^{2} \ldots n!^{2}}

From this, we then know

π ( n + 1 ) π ( n ) = ( n + 1 ) ! n + 2 n ! n + 1 0 ! 2 1 ! 2 n ! 2 0 ! 2 1 ! 2 ( n + 1 ) ! 2 = ( n + 1 ) n n ! \frac{\pi(n+1)}{\pi(n)} = \frac{(n+1)!^{n+2}}{n!^{n+1}} \cdot \frac{0!^{2} 1!^{2} \ldots n!^{2}}{0!^{2} 1!^{2} \ldots (n+1)!^{2}} = \frac{(n+1)^{n}}{n!}

Subsituting in n = 2001 n = 2001 gives our desired answer of 4002 \boxed{4002} .

Seriously level 5

devansh shringi - 6 years, 2 months ago
Adithya Hp
Mar 14, 2015

Please change question to find the lowest value of m+n as (2002,2002) and (2001,2001) are both valid solutions.

2002^2002/2002!=(2002^2001) 2002/((2001!) 2002)=2002^2001/2001!

So it is already written minimum value of m+n.Then what further queries do you have?No you are somewhere wrong and I can see a error in your solution too.

D K - 2 years, 10 months ago

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