Evaluate the surface integral SS f(x,y,z) ds using an explicit representation of the surface. S f(x,y,z)=4xy; S is the plane z=1-x-y in the first octant. Set up the surface integral for the given function f(x,y,z) over the given surface S. Integrate with respect to x first. Use increasing limits of integra SS f(x,y,z) ds = SSO dx dy S (Type exact answers.)