Square all three numbers. If the largest number squared is equal to the sum of the squares of the other two, then the numbers form a Pythagorean triple.
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Since a Pythagorean triple is three positive integers a, b, and c such that (a^2)+(b^2)=(c^2), first take the sum of the squares of the two legs and make an estimate. For example, 3, 4, and 5 is a Pythagorean triple since 9+16=25