1 2 + 3 2 + 5 2 + ⋯ + ( 2 n − 1 ) 2 2 2 + 4 2 + 6 2 + ⋯ + ( 2 n ) 2 < 1 . 0 1
Find the minimum positive integer n satisfying the inequality above.
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I got simply 2 n − 1 2 ( n + 1 ) < 1 . 0 1 which is a linear inequality with n > 1 5 0 . 5 .
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Maybe I made a calculation error somewhere?
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Numerator is equal to 4 times
1
2
+
2
2
+
3
2
+
⋯
+
n
2
.
Denominator is equal to
3
1
(
4
n
3
−
n
)
.
Simplify them and you get Alan's inequality.
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@Pi Han Goh – ohhh that's crafty. How come I didn't see that?!?!
150.5 is false because it should be an integer
Your numerator is wrong. Can you fix the solution?
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I see my error now. I'm going to rewrite the whole thing carefully. Sorry!
@Calvin Lin I think it's correct now.
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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This problem is mainly utilizing formula for sum of two squares.
x = 1 ∑ k x 2 = 6 k ( k + 1 ) ( 2 k + 1 )
Using substitution, we get:
k = 1 ∑ n ( 2 k − 1 ) 2 k = 1 ∑ n ( 2 k ) 2
k = 1 ∑ n ( 4 k 2 − 4 k + 1 ) 4 k = 1 ∑ n k 2
4 [ 6 n ( n + 1 ) ( 2 n + 1 ) ] − 4 [ 2 n ( n + 1 ) ] + n 4 [ 6 ( n ) ( n + 1 ) ( 2 n + 1 ) ] < 1 . 0 1
Factor out n
6 4 ( n + 1 ) ( 2 n + 1 ) − 2 4 ( n + 1 ) + 1 4 [ 6 ( n + 1 ) ( 2 n + 1 ) ] < 1 . 0 1
6 4 ( n + 1 ) ( 2 n + 1 ) − 3 ⋅ 4 ( n + 1 ) + 6 4 [ 6 ( n + 1 ) ( 2 n + 1 ) ] < 1 . 0 1
6 8 n 2 − 2 4 [ 6 ( n + 1 ) ( 2 n + 1 ) ] < 1 . 0 1
6 2 ( 2 n + 1 ) ( 2 n − 1 ) 4 [ 6 ( n + 1 ) ( 2 n + 1 ) ] < 1 . 0 1
Now, mass canceling yields:
2 n − 1 2 ( n + 1 ) < 1 . 0 1
Solving this linear inequality yields that n > 1 5 0 . 5 , and the next integer that is greater than that (since this involves summation notation, we can only accept positive integers) is 1 5 1