I am trying to approximate the solution of vi_vh to vi_vh = 1 / sqrt(vx_vh^2 + (vi_vh+vz_vh)^2) with the expression vi_vh = a * vz_vh^3 + b * vz_vh^2 + c * vz_vh + d + e * vz_vh * vx_vh^2 where b and d are zero The approximation is for the region -2<><>