题目

Have you grasped the information taught by your teacher in class? Have you understood what you saw on the screen? Maybe everyone has their own learning style. Generally speaking, we gather information with our minds and bodies in different ways, such as seeing, hearing and doing. Then our brains deals with that information, organizing it and connecting it to things we are already aware of. In other words, our brains will deal with the information in different ways. Sometimes we think in pictures or words. Sometimes we remember details or the big pictures. For different people, their l（2006•聊城）一家用电器开发公司研制出一种新型电子产品，每件的生产成本为18元，按定价40元出售，每月可销售20万件．为了增加销量，公司决定采取降价的办法，经市场调研，每降价1元，月销售量可增加2万件．（1）求出月销售量y（万件）与销售单价x（元）之间的函数关系式（不必写x的取值范围）；（2）求出月销售利润z（万元）（利润=售价-成本价）与销售单价x（元）之间的函数关系式（不必写x的取值范围）；（3）请你通过（2）中的函数关系式及其大致图象帮助公司确定产品的销售单价范围，使月销售利润不低于480万元．