# Number Theory GCD Question

1. Feb 13, 2015

### PsychonautQQ

1. The problem statement, all variables and given/known data
Show that gcd(a+b,a-b) is either 1 or 2. (hint, show that d|2a and d|2b)

2. Relevant equations
d = x(a+b)+y(a-b)

3. The attempt at a solution
so by the definition of divisibility:
a+b = dr
a-b = ds

adding and subtracting these equalities from eachother we can arrive at where the hint wanted us to conclude:
(r+s) = 2a/d
(r-s) = 2b/d

Trying to figure out what to do from here, having a hard time using the hint to restrict d to 1 or 2.

2. Feb 13, 2015

### Dick

You must have some condition on a and b. Like, are they relatively prime?

3. Feb 15, 2015

### Rellek

Hey there,

I think a really important concept that you might be forgetting is the principle of linearity with divisibility. Since your gcd divides both a+b and a-b, it is then also true that your gcd will divide ANY linear combination of these integers (assuming integer weights, of course).

Namely, instead of being completely general with divisibility, realize that you can manipulate the variables x and y in the expression d | (a+b)x + (a-b)y to get a form that is equally valid. I think that your book wants you to assume that a and b are coprime integers, as well.

So, why not try playing around and actually picking numerical values for x and y that can help eliminate either a or b?