Editing Economic surplus (section)

===Calculation from supply and demand===
The consumer surplus (individual or aggregated) is the area under the (individual or aggregated) demand curve  and above a horizontal line at the actual price (in the aggregated case, the equilibrium price).  If the demand curve is a straight line, the consumer surplus is the area of a triangle:

:<math>CS = \frac{1}{2} Q_{\mathrm{mkt}} \left( P_{\max} - P_{\mathrm{mkt}} \right),</math>

where ''P''<sub>mkt</sub> is the equilibrium price (where supply equals demand), ''Q''<sub>mkt</sub> is the total quantity purchased at the equilibrium price, and ''P''<sub>max</sub> is the price at which the quantity purchased would fall to 0 (that is, where the demand curve intercepts the price axis).   For more general demand and supply functions, these areas are not triangles but can still be found using [[integral]] calculus. Consumer surplus is thus the definite integral of the demand function with respect to price, from the market price to the maximum reservation price (i.e., the price-intercept of the demand function):

:<math>CS = \int^{P_{\max}}_{P_{\mathrm{mkt}}} D(P)\, dP, </math>

where <math>D(P_{\max}) = 0. </math> This shows that if we see a rise in the equilibrium price and a fall in the equilibrium quantity, then consumer surplus falls.