Error estimates for the mixed finite element solution of
4th order elliptic problems with variable coefficients, which,
in the particular case of aniso-/ortho-/isotropic plate bending problems,
gives a direct, simultaneous approximation to bending moment tensor
field $\Psi= (\psi_{ij})_{1 \le i,j \le 2}$
and displacement field
'u', have been developed considering the combined effect of
boundary approximation and numerical integration.