z= xln(x^2+y^2)∂z/∂x = ln(x^2+y^2) + [2x^2/(x^2+y^2)]= ln(x^2+y^2) + 2 - 2[y^2/(x^2+y^2)]∂^2z/∂x^2 = 2[x/(x^2+y^2)] + 4[xy^2/(x^2+y^2)^2]
