Pythagorean triple is a set of three numbers such that sum of the squares of two of them is equal to the square of the third.

In case of 4,5, 6 it is observed that 4² = 16

5² = 25

And 6² = 36

No sum of squares of any of the two is equal to the square of the third, hence 4,5, 6 do not form a Pythagorean triple.

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