1. The problem statement, all variables and given/known data [PLAIN]http://www.netbookolik.com/wp-content/uploads/2010/07/q1.png [Broken] 2. Relevant equations 3. The attempt at a solution I thought, in order to take derivative function must be cont. so it would be nice to check limf as (x,y) goes to (0, 0) but it did not seem to be a correct solution because appliying l'hopital gets harder and harder. If anyone can help me to solve this problem? Thanks!