Question

.隨著世界氣候大會(huì)于2009年12月7－18日在丹麥?zhǔn)锥几绫竟�根的召開，“低碳”概念風(fēng)靡全球。在“低碳”理念的引領(lǐng)下，某市為實(shí)現(xiàn)森林城市建設(shè)的目標(biāo)，在今年春季的綠化工作中，綠化辦計(jì)劃為某住宅小區(qū)購(gòu)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(gòu)買雪松、香樟的數(shù)量相等。
信息二：如下表：

設(shè)購(gòu)買雪松，垂柳分別為x株、y株。
(1).寫出y與x之間的函數(shù)關(guān)系式(不要求寫出自變量的取值范圍)；
(2).當(dāng)每株垂柳的批發(fā)價(jià)P等于30元時(shí)，要使這400株樹苗兩年后對(duì)該住宅小區(qū)的空氣凈化指數(shù)不低于90，應(yīng)怎樣安排這三種樹苗的購(gòu)買數(shù)量，才能使購(gòu)買樹苗的總費(fèi)用最低？最低的總費(fèi)用是多少元？
(3).當(dāng)每株垂柳批發(fā)價(jià)格P(元)與購(gòu)買數(shù)量y(株)之間存在關(guān)系P＝30－0.05y時(shí)，求購(gòu)買樹苗的總費(fèi)用W(元)與購(gòu)買雪松數(shù)量x(株)之間的函數(shù)關(guān)系式(不要求寫出自變量的取值范圍)，并求出購(gòu)買樹苗總費(fèi)用的最大值。

Answer 1

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