SOLUTION: Notice that any four numbers once drawn in a square on a calender have the property bc-ad=7 for example: 13 14 20 21 20x14-13x21 = 280-273 =7

Question 955734: Notice that any four numbers once drawn in a square on a calender have the property bc-ad=7
for example: 13 14
20 21
20x14-13x21
= 280-273
=7
This works for any four numbers drawn in a square if they are set out like:
a b
c d
so how do you mathematically prove that bc-ad=7 for any of the four numbers?

Answer by Fombitz(32388) (Show Source): You can put this solution on YOUR website!
Start with the upper left corner of the square.
Let's call that number, .
The one to the right would be, .
The one below would be .
And the one to the right of it would be .

So now you can solve the problem algebraically.

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