[MUSIC] So we've seen so far that when the firm is making positive profits or when price is greater than average total cost, we'll have entry into the industry. And when the firm's making negative profits or when price is less than average total cost, we'll have exit. That means that, in the long run, there's only one stable point, and that one is when price is exactly equal to the average total cost, because that is when profits are equal to zero. There's no incentive for firms to enter, and no incentive for firms to exit. So I want to point one thing out to you. In my graphs, I said that the long run will be at this price here. Not only as this price equal to the average total cost, but this is the case where the price is equal to the minimum of the average total cost. So let's just prove that, that must be correct. We have to put together two equations. The first equation is the profit maximizing equation which implies that the firm is always going to produce where price is equal to marginal cost. Again, that comes from profit maximization. The second equation is that in the long run, profits are going to be equal to zero, which means that price is equal to the average total cost. So if we take these two claims together, price is equal to marginal cost and price is equal to the average total cost, together this implies that it must the case that in the long run, the firm is producing what a marginal cost is equal to the average total cost. And is only one spot where the margin intersects the average total cost and that is at the minimum.