A coplanartrusssystem is shown in Figure. Point $2,3,4$ are fixed but the connected trusses are free to rotate around them so there is only tensile$/$compressive axial forces on the trusses. Coplanar means that the elements of the structure will only deform and displace in the x$-$y plane. Derive the expression of forces Qx and Qy as a function of displacements qx and qy using Castigliano$$s first theorem.