function    [Area, d, dd] = doubleconesurf(r1,r,r2,h1,h2)
%   doubleconesurf computes the surface area for a double conefrustum.
%   Returns the lateral surface area and partials wrt the middle radius.
[A1,x,d1,x,dd1] = conesurf (r1,r,h1);
[A2,d2,x,dd2,x] = conesurf (r,r2,h2);
Area = A1+A2 ;
d = d1+d2 ;
dd = dd1+dd2 ;
return