For the given truss system, neglect shear deformations and answer the followings:

a$)$ Calculate the internal forces of the truss elements.

b$)$ Assume $1kN$ vertical concentrated load moves through points A$-$B$.$ Plot the influence line diagrams of $O_2,O_4,O_6,V_3,V_4,D_4,U_2$ and $U_4.$

c$)$ Check the internal forces that have been determined at part $($a$)$ by influence lines. Make a comparison between part $($a$)$ and $($b$).$

d$)$ Calculate translational displacement $$ in terms of EA by using the virtual work principle. Assume all the elements of the truss system have the same cross$-$sectional area.