% Compute the derivative of shape functions respect REAL COORDINATES (x,y)
% INPUT: chi=[chi(1) chi(2)]=[ji nu] where the derivatives ...
% ... will be evaluated
% OUTPUT: is a matrix 4x2 containing the derivatives 'xyd':
% xyd=[dx_N1 dy_N1 ; dx_N2 dy_N2; dx_N3  dy_N3; dx_N4 dy_N4];
% Dependencies: it uses natural_deriv_2D4N and Jacobian_2D4N
function xyd=XY_deriv_2D4N(chi,xnod)
J=jacobian_2D4N(chi,xnod);
invJ=inv(J);
nd=natural_deriv_2D4N(chi);
xyd=nd*invJ;
return
end