Interesting Number 294

How many ordered pairs of positive integers ( a , b ) (a, b) are there such that

a 2 b 2 = 123 , 456 , 789 ? a^2 - b^2 = 123, 456, 789?


Inspiration

1 3 2 6

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3 solutions

We need a 2 b 2 = ( a + b ) ( a b ) = 123456789 a^2-b^2=(a+b)(a-b)=123456789

From this, we can say that a > b a>b .

Thus, the solutions would be of the type ( ( y + x ) / 2 , ( y x ) / 2 ) ((y+x)/2,(y-x)/2) , where x y = 123456789 xy=123456789 with y > x y>x

Factorizing 123456789 123456789 we find

123456789 = 3 2 × 3607 × 3803 123456789 = 3^2\times 3607 \times 3803

From which we can get six solutions for ( x , y ) (x,y) as ( 1 , 123456789 ) , ( 3 , 3 × 3607 × 3803 ) , ( 3 2 , 3607 × 3803 ) , ( 3607 , 3 2 × 3803 ) , ( 3803 , 3 2 × 3607 ) , ( 3 × 3607 , 3 × 3803 ) (1,123456789),\\ (3,3\times 3607\times 3803), \\ (3^2,3607 \times 3803),\\ (3607, 3^2 \times 3803),\\ (3803, 3^2 \times 3607),\\ (3 \times 3607, 3\times 3803)

There are six solutions for ( a , b ) (a,b)

( 61728395 , 61728394 ) , ( 20576133 , 20576130 ) , ( 6858715 , 6858706 ) , ( 18917 , 15310 ) , ( 18133 , 4330 ) , ( 11115 , 294 ) (61728395,61728394),\\ (20576133,20576130),\\ (6858715,6858706),\\ (18917,15310),\\ (18133,4330),\\ (11115,294)

Moderator note:

Great explanation! This is the typical approach for solving the generalized problem. The main issue is factorizing the number (which I will admit I resorted to Wolfram).

I suppose now the solution is a bit clearer.

Janardhanan Sivaramakrishnan - 5 years, 10 months ago

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Thanks!

Calvin Lin Staff - 5 years, 10 months ago

The changing of the incognitas makes the approach superiorly clear!!! #Nice

Cleres Cupertino - 5 years, 9 months ago
Revanth Gumpu
Aug 4, 2015

123456789 = 3^2 * 3607 * 3803. Thus the total number of factors are 12. Therefore the number of pairs are 12/2 which is 6 .
Is there a way to factorize the number without using wolframalpha? (I had to unfortunately use it)

Moderator note:

Why is it true that "Thus the total number of factors are 12. Therefore the number of pairs are 12/2"?

Halum Singh
Jul 30, 2015

(a + b)(a - b) = 123456789=3^2 X 3607 X 3803
So, number of divisors of 123456789 = (2 + 1)(1 + 1)(1 + 1) =12.
So, (a + b) can have 12 values. But as a, b are positive, a + b > a - b.
So, we get only 6 values of (a + b) and each value of (a + b) we get a unique value of
(a -b).
So, we get 6 ordered pairs (a, b).


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