- #1
PsychonautQQ
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- 10
Homework Statement
Let a and b be relatively prime. Show that gcd(a+b,ab) = 1
Homework Equations
ax+by = 1 for some integers x and y
The Attempt at a Solution
I set gcd(a+b,ab) = d. I'm now trying to show that d = 1 using elementary algebra techniques.
a+b = rd
ab = sd
ax + by = 1I'm kind of stuck here.. am I on the right track? Do I just need to aggresively rearrange stuff until I can express (a+b) and (ab) as a linear combination that equals one? or perhaps arrange them in such away that I show d divides both a and b individually and therefore it must be one since they are relatively prime? any hints?
Update:
a+b = rd ---> b = rd -a
ab = a(rd-a) = (ard)-(a^2) = sd
dividing both sides by d...
ar - (a^2 / d) = s
ar is an integer and s is an integer, so a^2 / d must be an integer, therefore d|a^2.
I employed a similar argument to show that d|b^2. Since d|a^2 and d|b^2 and gcd(a,b) = 1, we can use the fact that gcd(a^2,b^2) = 1 to conclude that d = 1.
Is this a solid argument?
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