The equivalent expression for and other similar terms in Eqn. (16.3), finally yields
where the geometrical parameters are
and
In Eqn.(17.3) are evaluated as the average of the four surrounding nodal values. Thus
Substitution into eqn. (17.3) generates the following nine-points discretisation of Eqn. (16.1):
and other similar terms in Eqn. (16.3), finally yields 
are evaluated as the average of the four surrounding nodal values. Thus 
