:Because all firms want to maximize profits, and profits are at their maximum when marginal profit = 0. But marginal profit (MP)= MR-MC, so when MP=0, then MR-MC=0, which means MR=MC. That is when total profits are at their maximum for a firm. | :Because all firms want to maximize profits, and profits are at their maximum when marginal profit = 0. But marginal profit (MP)= MR-MC, so when MP=0, then MR-MC=0, which means MR=MC. That is when total profits are at their maximum for a firm. |