The gains from trade made visible: willingness to pay above price, and price above cost.
Warming up the engine…
The gains from trade made visible: consumer surplus is what buyers would have paid minus what they did pay; producer surplus is price minus what sellers would have accepted.
CS = area below demand, above price ; PS = area above supply, below price
Every trade between someone who values a good more than it costs to make creates value out of thin air, and surplus measures exactly how much. Competitive equilibrium maximizes the total; taxes, price ceilings, and monopolies all shrink the pie, and the shrinkage (deadweight loss) is a triangle you can point to.
After any policy question, account for all four pieces: CS, PS, government revenue, and deadweight loss. Marks are usually attached to the DWL triangle and who bears the burden.
Now prove you have it
Move the curve to where you think it lands, and get told exactly which part you got right.
Real-world scenarios on this model
Equilibrium at Q 50, P 40.
Equilibrium: Q* = 50.0, P* = 40.0
Current equations
The market's win-win, made visible: consumers who'd have paid more than P* keep the blue triangle; producers who'd have sold for less keep the amber one. Together they're the total gains from trade this market creates.