Now, if we take a square of 16 numbers, then the general terms will be:
Diagonal sums: yx + (y+3)(x+3) = yx + y(x+3) + 3(x+3) = yx +yx +3y +3x +9 = 2yx +3y+3x+9
Y(x+3) + (y+3)x = yx +3y + yx +3x = 2yx + 3y + 3x
Clearly, the difference between the diagonal sums is equal to 9.
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