Investment has expanded the scale of production; Once the production volume is large, the fixed cost will be allocated to each piece, so the unit cost will be reduced. So the more investment, the lower the cost. However, the marginal utility of long-term additional investment in reducing costs will be reduced.

For example, suppose you run a laundry, and you have 1 washing machine, which can wash 500 pieces of clothes every month, with a charge of 5 yuan and a variable cost of 3 yuan. Every month, a fixed amount of protection fee of 1000 should be paid to Jie Qiang Ge Jiao. Then the allocation of protection fee to each piece is 1000/500=2 yuan, and the total cost of each piece is 3+2=5 yuan. To put it simply, you worked in vain and paid protection money for all the money you earned. Then, if you buy 1 washing machine (investment), you can wash 1 0,000 pieces every month. At this time, the protection fee is allocated to each block, 1, 000 = 1 yuan. At this time, the total cost of each garment is 3+ 1 = 4 yuan, and the cost is reduced. If you own three washing machines, the cost will be reduced to 3+ 1000/ 1500=3.67 yuan; If you have four washing machines, the cost will be reduced to 3+ 1000/2000=3.5 yuan. Do you see it? At this time, the effect of buying a new washing machine on reducing costs is getting smaller and smaller, and the marginal utility is decreasing.

Truth is universal, and examples are created by cudgels. Oh, get the integral:)