Problem $3[8$ points$]$ Using the direct stiffness method,

Distinguish the unknown and prescribed degrees of freedom $($displacement$($s$))$;

Express stiffness matrices for each element;

Write down the system of equations for unknown displacement and reactions and determine all unknown $($displacement$($s$)$ and reactions$)$;

Check that the reactions and external forces satisfy equilibrium equations;

Determine the internal forces in each element.

Consider the value $EA=300MNm.$