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This question already has an answer here:

Isn't 2^(0.5) rational?

Method for proving: Contradiction. So show not P:2^(0.5) is rational.

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marked as duplicate by Matthew Conroy, Chase Ryan Taylor, Community Feb 8 '18 at 7:08

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Let $7^{\frac 12} = a$ and $2^{\frac 12} = b$.

If you're allowed to assume (or you can show, and it's quite easy) that both $a$ and $b$ are irrational, then the problem becomes very easy.

Assume to the contrary that $a-b = X$ is rational.

Then $a+b = Y$ has to be irrational because $X+Y = 2a$ which is irrational (as assumed or previously shown).

Now consider $XY = (a+b)(a-b) = a^2 - b^2 = 7-2 = 5$.

This is rational. But the product of a rational ($X$, by assumption) and an irrational ($Y$, as deduced) number cannot be rational. We have arrived at a contradiction. Hence $X$ is irrational.

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