说明：  某厂向用户提供发动机，合同规定，第一、二、三季度末分别交货40台、60台、80台．每季度的生产费用为 （元），其中x是该季生产的台数．若交货后有剩余，可用于下季度交货，但需支付存储费，每台每季度c元．已知工厂每季度最大生产能力为100台，第一季度开始时无存货，设a=50、b=0.2、c=4，问工厂应如何安排生产计划，才能既满足合同又使总费用最低．讨论a、b、c变化对计划的影响，并作出合理的解释
(To provide users with a factory engine, contract, first, second and third quarter respectively, delivered at the end of 40, 60, 80. Quarterly cost of production (million), in which x is the quarter of the number of production. If the remaining after delivery, can be used for delivery next quarter, subject to payment of storage fees, for each element c of each quarter. The largest known plant production capacity of each quarter to 100, the beginning of the first quarter of stock, set up a = 50, b = 0.2, c = 4, asked how to arrange the production of the factory plans to meet the total cost of the contract makes the minimum . Discussion of a, b, c of the impact of change on the plan and make a reasonable explanation)