Number Theory: Simple Divisibility & GCD

[doubleaxel195]

Homework Statement

Prove that if N=abc+1, then (N,a)=(N,b)=(N,c)=1.

Homework Equations

The Attempt at a Solution

Assume N=abc+1. We must prove (N,a)=(N,b)=(N,c)=1. Proceeding by contradiction, suppose (N,a)=(N,b)=(N,c)=d such that $$d\not=1$$. Then we know, d | N and d | abc. Thus, from our assumption, we see that d | 1, a contradiction.

Is this a valid argument? Also, what is another way to prove this without using contradiction? Thanks, this is my first class with proofs. Also, I know "if (a,b)=d, then ax+by=d." Is the converse true?

 

[micromass]

Yes, this is a valid argument.

And the converse is not true: consider 9.1+7.(-1)=2, but (9,7) is not 2...

 

[doubleaxel195]

Thank you very much!