1. The problem statement, all variables and given/known data The problem is : For the functions from N*N --> N, determine if the following functions are surjective: f(a, b) = a + b f(a, b) = ab(b+1)/2 2. Relevant equations N is all natural numbers 3. The attempt at a solution My problem is I know the definition of surjective, but I don't know what to do with f(a, b) because I've never seen this before. THanks for the help!