Is this problem irrational or simply ab-surd?

Find the minimum value of n ϵ N n\epsilon N for which the expression n 1 + n + 2014 \sqrt{n-1}+\sqrt{n+2014} is rational.


The answer is 290.

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1 solution

Vishnu C
Apr 1, 2015

Let's say that n 1 + n + 2014 = x n \sqrt{n-1}+\sqrt{n+2014}={x}_{n} is rational for some n. So, 1 / x n 1/{x}_{n} should also be rational. n + 2014 n 1 2015 \Rightarrow\frac{\sqrt{n+2014}-\sqrt{n-1}}{2015} is also rational. Therefore, n + 2014 n 1 \sqrt{n+2014}-\sqrt{n-1} is also rational. As the sum of two rationals cannot be irrational, we find out that n 1 \sqrt{n-1} and n + 2014 \sqrt{n+2014} are rational. As n ϵ N , n\epsilon N, let for some x , y ϵ N x,y\epsilon N , n + 2014 = x 2 a n d n 1 = y 2 n+2014={x}^{2} \quad and \quad n-1 = {y}^{2} . x 2 y 2 = ( x y ) ( x + y ) = 2015 = 5 × 13 × 31. \Rightarrow {x}^{2}-{y}^{2}=(x-y)(x+y)=2015=5\times 13\times 31. 2015 has only 8 factors. This means that x+y=65, 155, 403, 2015.and x-y=31, 13, 5, 1. Solving these equations, we can get that minimum value of y=17. n = 289 + 1 = 290 \\ \therefore n=289+1=\boxed{290}

In response to Vishnu C : Can you p l e a s e please explain me it in a e a s y easy way !!!!!!!

Abhisek Mohanty - 6 years, 2 months ago

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vishnu c - 6 years, 2 months ago

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