- #1
natasha13100
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Homework Statement
Show that lim(n→infinity) 1/n*{2*4*...*2n/(1*3*...*(2n-1))}^2 exists without finding the limit.
Homework Equations
probably the following:
Let {xn} be a sequence such that xn≥xn+1 and xn≥M for every n. Then the series is convergent.
The Attempt at a Solution
I know I need to do a proof.
n≥1 (n cannot be 0 because 1/n is undefined and n must be an integer since it's the term number)
{xn}2=22/1*42/32*...*(2n)2/(2n-1)2=2/1*(2*4)/32*(4*6)/52*...*((2n-2)*2n)/(2n-1)2*2n
for any integer n>1, ((2n-2)*2n)/(2n-1)2*2n=((2n-1)2-1)/(2k+1)2<1
Therefore, {xn}2<4n and 1/n*{xn}2<4
(Hence, I can prove n≥1 and n≤4 (for n=1, 1/1*{2/1)^2=4 so I included 4)
for n=1 1/1*{2/1}^2=4 and for n=2 1/2*{2*4/(1*3)}^2=64/18=32/9
thus, n=1>n=2 establishing a basis for an induction proof
assume for n=k that 1/k*{2*4*...*2k/(1*3*...*(2k-1))}^2>1/(k+1)*{2*4*...*2(k+1)/(1*3*...*(2k))}^2 so that 1/n*{2*4*...*2n/(1*3*...*(2n-1))}^2>1/(n+1)*{2*4*...*2(n+1)/(1*3*...*(2n))}^2
This is where I am stuck. I want to show that the statement 1/n*{2*4*...*2n/(1*3*...*(2n-1))}^2>1/(n+1)*{2*4*...*2(n+1)/(1*3*...*(2n))}^2 holds true for n=k+1 but I can't figure out how to get to that point. I know you can't just replace the n's with k+1's in the proof.