In finite-element calculations the bilinear function

is commonly used for interpolation over a quadrilateral and data is always stored in matrix form. If the function fits the data u(0, 0) = u₁, u(p, 0) = u₂, u(0, q) = u₃ and u(p, q) = u₄ at the four corners of a rectangle, use matrices to find the coefficients a, b, c and d.