[MUSIC] Let's spend just a couple more minutes thinking about these cost curves. So again, the total cost is comprised of fixed costs and variable costs. So this average total cost curve is the average of the fixed costs plus the average of the variable costs. The average of the fixed cost is always falling because it's the fixed cost divided by quantity and the bigger is the quantity, the lower are the average fixed cost. But this variable cost is more complicated, because the variable cost increase with quantity, so we're taking something that's increasing by quantity and dividing it by quantity. But our marginal cost helps us think about this relationship. As long as the margin is below the average, it must be pulling the average down. As I tell my students, if you get a grade that's below your GPA, below your grade point average, your grade point average will be dropping. And once the margin is above the average, it will be pulling the average up. Again, you get a grade that's higher than your average grade, your average grade will be increasing. So let's summarize. We've looked at two costs, the marginal cost and the average total cost. We know the general shape of the marginal cost, the general shape of the average total cost and the relationship between the two. When the margin is below the average, it pulls the average down. When the margin is above the average, it pulls the average up. I think we're ready to think about Firm's profit maximization.