Suppose it were rational, then you could write it as the ratio n/m where n and m are relatively prime integers (ie they don't have any factors in common), and m>n.Then by squaring both sides you would get
n^2/m^2 = 2/3. So 3n^2 = 2m^2
The right hand side must be an even integer (because multiplied by 2), so therefore the left hand side must be as well. Since 3 is odd, n^2 must be even, which means n must be even (the square of an odd number is odd). If n is even we can write n = 2p, where p is another integer. Hence we would have. 3*4p^2 = 2m^2. or
2*3p^2 = m^2
Now the left hand side is clearly even, so therefore m^2 must also be even, hence m must be even.
But if m and n are both even, they can both be divided by 2. This contradicts our original assumption that n and m are relatively prime. We cannot have a contradiction, hence our original assumption that sqrt(2/3) is rational must be wrong.
sqrt(2/3) is irrational
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