let f:NxN->N be defined by f(x,y)=(1/2)(x+y)(x+y-1)-(1-y)

Is f one-to-one? Does f map (NxN) to N or onto N (meaning are all values in N in the range of f or is the range a subset of N)?

Can I get a hint on how to go about answering either of those questions. I know what each of the questions mean, but I have no idea how to answer them.