Take nine numbers forming a square in a calendar and mark the four numbers at the corners. Then the difference between the squares of the diagonal pairs is always ____.
56
Let the number at the top left corner of the square of nine numbers in a calendar be 'x'.
Then such a square in the calendar would be of the form:
xx+1x+2x+7x+8x+9x+14x+15x+16
The four numbers at the corners are x, x+2, x+14, and x+16, in which x and x+16 form one diagonal pair and x+2 and x+14 form another diagonal pair.
Now, x2+(x+16)2 = x2+(x2+32x+256) = x2+x2+32x+256 = 2x2+32x+256
and (x+14)2+(x+2)2 = (x2+28x+196)+(x2+4x+4)=x2+28x+196+x2+4x+4 = 2x2+32x+200
So, (x2+(x+16)2)−((x+14)2+(x+2)2) = (2x2+32x+256)−(2x2+32x+200) = 2x2+32x+256−2x2−32x−200 = 56