How to find forces in truss members FG, GD, CD using virtual work

[Tor]

1. Calculate the forces in the members FG, GD, and CD of the truss shown in the figure by using the principle of virtual work. All horizontal and vertical members are 1m long
2. I have attached the diagram
3. I can easily calculate this using normal principles, but not understanding how to use Virtual work

 

[paisiello2]

Find the relationship between the virtual displacements caused by the loads and the virtual displacements at the joints.