I have already done the proof, but I just want to make sure it is correct!
f: ZxZ -> Z f(m,n)=m+n
Prove that f is onto using the definition of onto.
Suppose that f(m,n)=m+n.
Then, f(m,n)=a iff m+n=a iff m=a-n which is in Z.
Therefore, f(m+n)=m+n is onto.
Is this correct? Or proper?