1. The problem statement, all variables and given/known data find the angle between the normals to the surface xy=z2 at the points (1,4,2) and (-3,-3,3) 2. Relevant equations none 3. The attempt at a solution del S = 2zy i + 2zx j and at the two points, del S = 16i+4j and del S = -18i-18j using the dot product, i got cos theta = -5/(4*sqrt 34) which is NOT the answer. Is my partial derivative equation right ??? Please help.