function y = BilinearQuadElementArea(x1,y1,x2,y2,x3,y3,x4,y4)
%BilinearQuadElementArea   This function returns the area 
%                          of the bilinear quadrilateral 
%                          element whose first node has 
%                          coordinates (x1,y1), second 
%                          node has coordinates (x2,y2), 
%                          third node has coordinates 
%                          (x3,y3), and fourth node has
%                          coordinates (x4,y4) .
yfirst = (x1*(y2-y3) + x2*(y3-y1) + x3*(y1-y2))/2;
ysecond = (x1*(y3-y4) + x3*(y4-y1) + x4*(y1-y3))/2;
y = yfirst + ysecond;