Date: Tue, 20 Jul 1999 21:10:07 -0700
Sender: Michael & Stephanie

I am a college student but this problem is for high school math.

if you have the equation x= n2 - m2 (ie 40= 72-32= 49-9) x must = a positive number

1) which squared numbers work as n and m
2) how does it work
3) if my teacher gave me the number for x; how could I figure out this problem

Thanks for your help

The pattern that you need to see here is the difference of squares. That is for any numbers m and n

x = m2 - n2 = (m - n )(m + n)

Notice that (m - n) + (m + n) = 2m which is even. Hence if x can be written as the difference of two squares then it must be possible to write x as the product of two numbers whose sum is even.
   Numbers x that are divisible by 2 but not by 4, such as 6 or 30, cannot be written as the difference of two squares since if you write such a number x as a product, say k times j then one of k and j is even and the other is odd so k + j is odd. On the other hand if x is divisible by 4 then you can write x = 2(2s) for some number s and 2 + 2s is even. If x is odd and you write x as a product of k and j then both k and j are odd so k + j is even.
   Now suppose that x is k times j and k + j is even. Let m = (k + j)/2. You should now be able to find n so that

x = (m - n )(m + n) = m2 - n2

   Thus the only integers x for which you cannot find integers m and n such that x = m2 - n2 are integers that are divisible by 2 but not by 4.


Go to Math Central

To return to the previous page use your browser's back button.