I'm still not that brilliant at proving things and i'm having trouble with this lemma.Lemma: Let a and n be integers with hcf(a,n)=1. Then

n divides ab if and only if n divides b.

I started with

Suppose n divides ab

Combining both expressions gives: .

This is an integer multiple of b so n must divide b.

Is this right?

I'm also having trouble with the converse of the statement as well. I'll post any progress I make.