Is there more than one?

Number Theory Level pending

The ordered pair of four-digit numbers ( 2025 , 3136 ) (2025,3136) has the property that each number in the pair is a perfect square and each digit of the second number is 1 1 more than the corresponding digit of the first number.

Find the sum of all ordered pairs of five-digit numbers with the same property.

if you find two pairs ( a , b ) (a,b) and ( c , d ) (c,d) submit the answer as a + b + c + d . a+b+c+d.


The answer is 37561.

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1 solution

Cantdo Math
May 1, 2020

let k 2 k^2 be the first of the ordered pairs,then the problem condition says that

k 2 + 11111 = x 2 k^2+11111=x^2 for some integer x x or equivalently ( x + k ) ( x k ) = 11111 (x+k)(x-k)=11111 .Now,since 11111 = 1 11111 = 41 271 11111=1*11111=41*271 .these are the only possible ways you could factor 11111 11111 into two natural numbers.

we have k = ( 271 41 ) 2 k=\frac{(271-41)}{2} or k = ( 11111 1 ) 2 k=\frac{(11111-1)}{2} .First one gives us solution: ( 13225 , 24336 ) (13225,24336) .The latter k is too big for k 2 k^2 to be a 5 5 -digit number.

Hence,our answer is 37561 \boxed{37561}

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