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# Sums of Squares

NOTES AND BACKGROUND

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Age 16 to 18

Challenge Level

This is a problem by Lewis Carroll and it revolves around some of the many interesting properties of sums of squares of integers. Is it always true that if you double the sum of two squares you get the sum of two squares? If so can you prove it? Here are some examples.

$2(5^2 + 3^2) = 2(25 + 9) = 68 = 64 + 4 = 8^2 + 2^2$

$2(7^2 + 4^2) = 2(49 + 16) = 130 = 121 + 9 = 11^2 + 3^2$

NOTES AND BACKGROUND

In his book Pillow-Problems Lewis Carroll extends this idea with a further problem. Prove that 3 times the sum of three squares is also the sum of 4 squares.

For further problems like this see Lewis Carroll's Games and Puzzles compiled by Edward Wakeling published by Dover Books ISBN 0-486-26922-1.