The integral over the surface of the box is equal to the sum of the integrals over eash of the six sides. First let’s calculate the integral over the side at x+∆x. This side is perpendicular to the unit vector x. If we assume that the area of this side is small (that is, we assume that ∆y and ∆z are small) then we can approximate the integral over this side by the value of the integrand at the middle of the side times the area of the side. The value of the integrand F n at this midpoint is