Question

.隨著世界氣候大會于2009年12月7－18日在丹麥首都哥本哈根的召開，“低碳”概念風靡全球。在“低碳”理念的引領下，某市為實現森林城市建設的目標，在今年春季的綠化工作中，綠化辦計劃為某住宅小區購買并種植400株樹苗，某樹苗公司提供如下信息：
信息一：可供選擇的樹苗有雪松、香樟，垂柳三種，并且要求購買雪松、香樟的數量相等。
信息二：如下表：

設購買雪松，垂柳分別為x株、y株。
(1).寫出y與x之間的函數關系式(不要求寫出自變量的取值范圍)；
(2).當每株垂柳的批發價P等于30元時，要使這400株樹苗兩年后對該住宅小區的空氣凈化指數不低于90，應怎樣安排這三種樹苗的購買數量，才能使購買樹苗的總費用最低？最低的總費用是多少元？
(3).當每株垂柳批發價格P(元)與購買數量y(株)之間存在關系P＝30－0.05y時，求購買樹苗的總費用W(元)與購買雪松數量x(株)之間的函數關系式(不要求寫出自變量的取值范圍)，并求出購買樹苗總費用的最大值。

Answer 1

（1） ·············································································································· 2分
（2）由題意得：   解得········································ 5分
0
當x=200時，=10000元························································································ 7分
（3）
=···················································································· 9分
當x=175時，元··················································································· 10分

解析