Q.The demand function for a commodity is $p=20-2x$ and the supply function is $p=2+x$ (where $x$ is quantity and $p$ is price in rupees). Find the equilibrium quantity and price, and hence compute the consumer's surplus and the producer's surplus.

Market equilibrium is where the demand and supply prices are equal; consumer's and producer's surplus are then found by integrating each curve up to the equilibrium quantity.

Step 1 — find equilibrium by equating demand and supply prices:

Step 2 — consumer's surplus:

$= (120-36)-0 - 48 = 84-48 = 36$ …

Using the WRONG curve in each surplus formula — computing consumer's surplus from the supply curve, or producer's surplus from the demand curve, instead of demand for CS and supply for PS. Also, forgetting to subtract (or add) the rectangle $p_{0}x_{0}$, w …