Key Idea: Topic 2.3 shows how **demand and supply interact** to determine the market price and quantity, what happens when equilibrium **changes**, and how to measure **consumer and producer surplus**.
✅ Core definitions
⚖️ Surplus and shortage
📊 How equilibrium changes
Memory trick: D shifts → P and Q move in the **same direction**. S shifts → P and Q move in **opposite directions**.
🔀 Simultaneous shifts
Always draw the diagram step by step: original equilibrium → shift one curve → shift the other → identify what is certain and what is indeterminate.