If ∇f=⟨2x,2y⟩\nabla f = \langle 2x, 2y \rangle∇f=⟨2x,2y⟩, what is the function f(x,y)f(x, y)f(x,y) (up to a constant)?
2x+2y2x+2y2x+2y
x2+y2x^2+y^2x2+y2
x+yx+yx+y
x2y2x^2y^2x2y2