Make it solvable

a, b, and x are positive integers such that

a + b = x \sqrt{a} + \sqrt{b} = \sqrt{x}

How many possible values of x less than or equal to 1000 are there?


The answer is 392.

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1 solution

Jeremy Galvagni
Mar 25, 2018

This is solvable if and only if x x has a perfect square factor. Proof:

x = a + b + 2 a b x = a + b + 2\sqrt{ab} so a b ab must be a square. So if a has an odd powered factor then b has the same odd powered factor.

a \sqrt{a} simplifies to a number of form d c d*\sqrt{c} , this implies b \sqrt{b} simplifies to e c e*\sqrt{c} therefore

x = d c + e c = ( d + e ) c \sqrt{x} = d*\sqrt{c} + e*\sqrt{c} = (d+e)*\sqrt{c}

x = ( d + e ) 2 c x = (d+e)^{2}*c

so x x has a perfect square factor.

Counting the number of integers to 1000 which have a square factor takes awhile but is trivial. There are 392 392 of them.

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