Find the function f (x,y) whose differential is

df = (x + y)⁻¹dx + (x + y)⁻¹dy

and which has the value f(1,1) = 0. Do this by performing a line integral on a rectangular path from (1,1) to (x₁,y₁) where x₁ > 0 and y₁ > 0.