Henry Ernest Dudeney/Puzzles and Curious Problems/129 - Squares and Cubes/Solution

by : $129$

 * Squares and Cubes
 * Find two different numbers such that the sum of their squares shall equal a cube, and the sum of their cubes equals a square.

Solution
We have:


 * $625^2 + 1250^2 = 125^3$

while:
 * $625^3 + 1250^3 = 46 \, 875^2$

Proof
states that if one number is $625 m^6$ and the other number double the first, you can get any number of solutions of a particular series.

The answer given is for $m = 1$.

He probably exploited the fact that $1 + 2^3 = 3^2$.

Therefore for any $k$:
 * $k^3 + \paren {2 k}^3 = 9 k^3$

$9 k^3$ would be a square if $k$ is itself a square.

So we write $k = n^2$:
 * $\paren {n^2}^3 + \paren {2 n^2}^3 = \paren {3 n^3}^2$

and we see that the second equation is satisfied.

Substituting this to the first equation, we have:
 * $\paren {n^2}^2 + \paren {2 n^2}^2 = 5 n^4$

If $5 n^4$ is a cube, $n$ must be divisible by $5$.

By writing $n = 5 a$, we still require $5^5 a^4$ to be a cube.

Hence $a$ must again be divisible by $5$.

By writing $a = 5 b$, we only require $5^9 b^4$ to be a cube.

Thus $b$ must itself be a cube.

Finally we write $b = m^3$.


 * $k = n^2 = 25 a^2 = 625 b^2 = 625 m^6$

and we see that:
 * $\paren {625 m^6}^2 + \paren {1250 m^6}^2 = 5^9 m^{12}$

which is a cube, so the first equation is also satisfied.