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Question

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 ____.


A

56

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B

24

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C

28

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D

12

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Solution

The correct option is A

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+2562x232x200 = 56


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