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Consider next the industry equilibrium. Since the industry supply is the sum of the individual supplies, and the industry demand curve is the sum of individual demands, an equilibrium price and
quantity (
,
) are defined by the intersection of these industry-level curves, as in Figure 9.4. Here, each firm takes 
as given (it is so small that it cannot influence the going price), and supplies an
amount determined by the intersection of this price with its MC curve. The sum of such quantities is therefore 
.
Figure 9.4 Market equilibrium
The market supply curve S is the sum of each firm’s supply or MC curve above the shut-down price. D is the sum of individual demands. The market equilibrium price and quantity are defined
by 
and 
.
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