Crazy Minimum

Algebra Level 5

2 2 + 4 2 + 6 2 + + ( 2 n ) 2 1 2 + 3 2 + 5 2 + + ( 2 n 1 ) 2 < 1.01 \large \dfrac{2^2+4^2+6^2+\cdots+(2 n)^2}{1^2+3^2+5^2+\cdots+(2 n-1)^2}<1.01

Find the minimum positive integer n n satisfying the inequality above.

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The answer is 151.

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2 solutions

Hobart Pao
Apr 10, 2016

This problem is mainly utilizing formula for sum of two squares.

x = 1 k x 2 = k ( k + 1 ) ( 2 k + 1 ) 6 \displaystyle \sum_{x=1}^{k} x^2 = \dfrac{k(k+1)(2k+1)}{6}

Using substitution, we get:

k = 1 n ( 2 k ) 2 k = 1 n ( 2 k 1 ) 2 \dfrac{\displaystyle \sum_{k=1}^{n} (2k)^2 }{\displaystyle \sum_{k=1}^{n} (2k-1)^2 }

4 k = 1 n k 2 k = 1 n ( 4 k 2 4 k + 1 ) \dfrac{\displaystyle 4 \sum_{k=1}^{n} k^{2} }{\displaystyle \sum_{k=1}^{n} \left( 4k^2 - 4k + 1 \right) }

4 [ ( n ) ( n + 1 ) ( 2 n + 1 ) 6 ] 4 [ n ( n + 1 ) ( 2 n + 1 ) 6 ] 4 [ n ( n + 1 ) 2 ] + n < 1.01 \dfrac{4 \left[ \dfrac{(n)(n+1)(2n+1)}{6} \right] }{4 \left[ \dfrac{n(n+1)(2n+1)}{6} \right] - 4 \left[ \dfrac{n(n+1)}{2} \right] + n } < 1.01

Factor out n n

4 [ ( n + 1 ) ( 2 n + 1 ) 6 ] 4 ( n + 1 ) ( 2 n + 1 ) 6 4 ( n + 1 ) 2 + 1 < 1.01 \dfrac{4 \left[ \dfrac{(n+1)(2n+1)}{6} \right]}{\dfrac{4 (n+1)(2n+1)}{6} - \dfrac{4(n+1)}{2} + 1} < 1.01

4 [ ( n + 1 ) ( 2 n + 1 ) 6 ] 4 ( n + 1 ) ( 2 n + 1 ) 3 4 ( n + 1 ) + 6 6 < 1.01 \dfrac{4 \left[ \dfrac{(n+1)(2n+1)}{6}\right] }{\dfrac{4(n+1)(2n+1) - 3 \cdot 4(n+1) + 6}{6}} < 1.01

4 [ ( n + 1 ) ( 2 n + 1 ) 6 ] 8 n 2 2 6 < 1.01 \dfrac{4 \left[ \dfrac{(n+1)(2n+1)}{6}\right] }{\dfrac{8n^{2} - 2}{6}} < 1.01

4 [ ( n + 1 ) ( 2 n + 1 ) 6 ] 2 ( 2 n + 1 ) ( 2 n 1 ) 6 < 1.01 \dfrac{4 \left[ \dfrac{(n+1)(2n+1)}{6}\right] }{\dfrac{2 (2n+1)(2n-1) }{6}} < 1.01

Now, mass canceling yields:

2 ( n + 1 ) 2 n 1 < 1.01 \dfrac{2(n+1)}{2n-1} < 1.01

Solving this linear inequality yields that n > 150.5 n>150.5 , and the next integer that is greater than that (since this involves summation notation, we can only accept positive integers) is 151 \boxed{151}

I got simply 2 ( n + 1 ) 2 n 1 < 1.01 \dfrac{2(n+1)}{2n-1}<1.01 which is a linear inequality with n > 150.5 n>150.5 .

Alan Enrique Ontiveros Salazar - 5 years, 2 months ago

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Maybe I made a calculation error somewhere?

Hobart Pao - 5 years, 2 months ago

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Numerator is equal to 4 times 1 2 + 2 2 + 3 2 + + n 2 1^2 + 2^2 + 3^2 + \cdots + n^2 .
Denominator is equal to 1 3 ( 4 n 3 n ) \dfrac13 (4n^3-n) .
Simplify them and you get Alan's inequality.

Pi Han Goh - 5 years, 2 months ago

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@Pi Han Goh ohhh that's crafty. How come I didn't see that?!?!

Hobart Pao - 5 years, 2 months ago

150.5 is false because it should be an integer

Youssef Saad - 5 years, 2 months ago

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That's the inequality statement

Hobart Pao - 5 years, 2 months ago

Your numerator is wrong. Can you fix the solution?

Calvin Lin Staff - 5 years, 2 months ago

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I see my error now. I'm going to rewrite the whole thing carefully. Sorry!

Hobart Pao - 5 years, 2 months ago

@Calvin Lin I think it's correct now.

Hobart Pao - 5 years, 2 months ago

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Looks good. Thanks!

Calvin Lin Staff - 5 years, 2 months ago
Ashutosh Sharma
Mar 26, 2018

rewrite it as {100/101} < {summation squares of odd terms till 2n-1 / summation of squares of even term till 2n) . now interesting part comes here is to use componendo property so that in numerator we are left with summation of square till 2n tems and in denominator 4 x sum of squares of n terms . now use summation of square formula on solving we get simple equation as *(201/101)< [(4n+1)]/[2(2n+1)] * which results 1 50.5<n and thus ans is 151.

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