1. The problem statement, all variables and given/known data Show that f(x,y)=g(x,y)h(x,y) is stationary if and only if: g=0 and h=0 OR df/dx=0 and (dg/dx)(dh/dy)=(dg/dy)(dh/dx) (All the d's in the line above should be curly d's for partial derivatives.) 2. Relevant equations 3. The attempt at a solution I tried expressing df as a total derivative then setting this equal to zero, but I wasn't getting anywhere.