When the output increases from 400 to 500, according to the formula, the marginal cost is 500,000 yuan, and when the output increases from 500 to 600, the marginal cost is 520,000 yuan, so the total cost increment is 50+52 = 654.38+0.02 million yuan. When marginal cost = average cost, the average cost reaches the lowest, and the total cost is fixed cost+variable cost. That is 36+2 *1+40+2 * 2+40+2 * 3+40+... = x (x+ 1)+40 * x+36, and the average cost is (x (x+1)+. The result of the root sign should be 6.5. The result may be wrong, but the idea should be right. I hope I can help you.