1. **Problem statement:** Calculate and plot the internal force diagrams (shear force and bending moment) of the given beam structures using the force method (Phương pháp PTHH) with constant flexural rigidity $EI$. 2. **Method and formulas:** - The force method involves releasing redundant supports and applying compatibility conditions. - For beams under distributed load $q$, point load $P$, and moment $M$, use equilibrium equations and moment-area theorems. - Important formulas: - Shear force due to uniform load $q$ over length $L$: $V = qL/2$ at supports. - Moment due to uniform load: $M = qL^2/8$ at mid-span for simply supported beams. - Effect of point load $P$ and moment $M$ applied at spans. 3. **Diagram a):** - Beam spans: $3m$, $a$, $a$, $a$ with loads $q$ on $3m$, $P$ at first $a$, $M$ at second $a$. - Calculate reactions by equilibrium. - Use force method to find redundant reactions. - Calculate shear force and bending moment diagrams stepwise. 4. **Diagram b):** - Cantilever beam length $2a$ with distributed load $q$ and moment $M$ at fixed end, roller at $3a/2$. - Calculate fixed end moments and shear. - Use compatibility to find reactions. - Draw internal force diagrams. 5. **Diagram c):** - Beam with spans $a$, $a$, $a$, $3m$ with loads $q$, $P$, $M$ on first three spans respectively. - Calculate reactions using force method. - Determine shear and moment diagrams. 6. **Summary:** - Each beam requires setting up equilibrium equations. - Apply force method to solve for unknown reactions. - Calculate internal forces at key points. - Plot diagrams accordingly. **Final note:** Detailed numeric solutions require values for $a$, $q$, $P$, $M$ which are not provided.