Consecutive Integers?

How many pairs of integer solutions ( m , n ) (m,n) exist such that m 3 = 8 n 3 10 n 2 n + 2 m^3 = 8n^3 - 10n^2 - n + 2 ?


The answer is 2.

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2 solutions

Anson Lee
Mar 24, 2019

Suppose that m > 2 n 1 m > 2n - 1 .Then,

( 2 n 1 ) 3 < 8 n 3 10 n 2 n + 2 2 n 2 7 n + 3 > 0 ( 2 n 1 ) ( n 3 ) > 0 n < 1 2 , n > 3 (2n - 1)^3 < 8n^3 - 10n^2 - n + 2 \\ \iff 2n^2 - 7n +3 > 0 \\ \iff (2n - 1)(n - 3) > 0 \\ \iff n < \frac{1}{2}, n > 3 .

Also suppose that m < 2 n m < 2n . Then,

( 2 n ) 3 > 8 n 3 10 n 2 n + 2 10 n 2 + n 2 > 0 ( 5 n 2 ) ( 2 n + 1 ) > 0 n < 1 2 , n > 2 5 . (2n)^3 > 8n^3 - 10n^2 - n + 2 \\ \iff 10n^2 + n -2 > 0 \\ \iff (5n - 2)(2n + 1) > 0 \\ \iff n < -\frac{1}{2}, n > \frac{2}{5}.

Therefore 2 n 1 < m < 2 n 2n -1 < m < 2n and so m m cannot be an integer; unless 1 2 n 3 \frac{1}{2} \leq n \leq 3 and n < 1 2 or n > 2 5 n < -\frac{1}{2} \text{ or } n > \frac{2}{5} , which gives n = 1 , 2 , 3 n = 1, 2, 3 , giving the pairs ( 1 , 1 ) (-1, 1) and ( 5 , 3 ) (5, 3) ( n = 2 n = 2 does not give an integer solution).

The only other possibility is when n < 1 2 or n > 3 n < \frac{1}{2} \text{ or } n > 3 and 1 2 n 2 5 -\frac{1}{2} \leq n \leq \frac{2}{5} , which gives only n = 0 n = 0 which yields no integer solutions for m m .

Thus the only pairs of solutions are ( 1 , 1 ) and ( 5 , 3 ) \boxed{(-1, 1) \text{ and } (5, 3)} .

Kushal Bose
Aug 1, 2016

Two solutions are (-1,1) and (5,3)

First one i have found by putting m= -n

Second one by Hit trial method

Can anyone give a proper wayout

So, how did you conclude that ( 1 , 1 ) (-1, 1) , ( 5 , 3 ) (5, 3) are only pairs of solution?

Ankit Nigam - 4 years, 10 months ago

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Truly speaking I have found answer is two

Kushal Bose - 4 years, 10 months ago

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