We construct market demand curves for products by horizontally summing quantities demanded of a good by each person in the market. So, if there are two people in the market for good , at a certain price, , individual 1 will demand and individual 2 will demand . We add up , and this gives us the total quantity demanded at the market at .
Market demands will be represented by capital letters from now on, and individual demands by lower case letters.
This lecture introduces the concept of elasticity.