We want to look at the areas of parallelograms that are tangent to the surface, in particular. Let vec(r)ᵤ = (xᵤ(u₀, v₀), yᵤ(u₀, v₀), zᵤ(u₀, v₀) and vec(r)ᵥ = (xᵥ(u₀, v₀), yᵥ(u₀, v₀), zᵥ(u₀, v₀)). Then, the parallelogram that is tangent to the surface at the point (u₀, v₀, r(u₀, v₀)) will have adjacent sides vec(r)ᵤ Δu and vec(r)ᵥ Δv. Explain why it makes sense that these vectors will make up the edges of the little parallelogram tangent to the surface.