Let the first number in the square be x, the others can be filled in as below,
Diagonal products: x (x+6) = x2 + 6x
(x+5)(x+1) = x(x+1) + 5(x+1) = (x2 + x) + (5x +5) = x2 + 6x + 5
Clearly, the difference between the two diagonal products is always 5.
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