Inequalities

Algebra Level 4

Find the smallest positive integer n n such that n n 1 < 0.01 \sqrt{n} - \sqrt{n-1} < 0.01 .


The answer is 2501.

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4 solutions

Kay Xspre
Aug 5, 2015

Re-arranging the equation gives n 0.01 < n 1 \sqrt{n}-0.01 < \sqrt{n-1} , then squared to n 0.02 n + 0.0001 < n 1 n-0.02\sqrt{n}+0.0001 < n-1 2 n < 100.01 -2\sqrt{n} < -100.01 n > ( 50.005 ) 2 n > (50.005)^{2} n > 2 , 500.500025 n > 2,500.500025 Therefore the smallest positive integer is n = 2501 n=2501

did the same but entered 2500 as answer and got it wrong.i am just too dumb

Kaustubh Miglani - 5 years, 8 months ago
Sathvik Acharya
Dec 28, 2017

Hint: Observe that n n 1 = 1 n + n + 1 \sqrt{n}-\sqrt{n-1}=\large \frac{1}{\sqrt{n}+\sqrt{n+1}} .

Zach Bian
Dec 29, 2017
  • Press F12 to open your browser's console.
  • Paste the following snippet into the console:
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let n=1;
while( Math.sqrt(n) - Math.sqrt(n++-1) >= .01 ){}
console.log(n-1);

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2501

Arkodipto Dutta
Mar 4, 2016

same method

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