问题 解答题
有一个湖泊受污染,其湖水的容量为V立方米,每天流入湖的水量等于流出湖的水量.现假设下雨和蒸发平衡,且污染物和湖水均匀混合.用g(t)=
p
r
+[g(0)-
p
r
]e-
r
v
t
(p≥0)
,表示某一时刻一立方米湖水中所含污染物的克数(我们称其湖水污染质量分数),g(0)表示湖水污染初始质量分数.
(1)当湖水污染质量分数为常数时,求湖水污染初始质量分数;
(2)分析g(0)<
p
r
时,湖水的污染程度如何.
答案

(1)设0≤t1<t2

因为g(t)为常数,g(t1)=g(t2),即[g(0)-

p
r
][e-
r
v
t1
-e-
r
v
t2
]=0,则g(0)=
p
r

(2)设0<t1<t2,g(t1)-g(t2)=[g(0)-

p
r
][e-
r
v
t1
-e-
r
v
t2
]

=[g(0)-

p
r
]•
e
r
v
t2
-e
r
v
t1
e
r
v
t1+t2

因为g(0)-

p
r
<0,0<t1<t2,g(t1)<g(t2).

故随着时间的推移,湖水污染越来越严重.

阅读理解

You often hear people refer to Africa as a country, not a continent. It is often a region that is described as being unified around poverty, disease and despair. Africa is far from being a homogenized continent. It has more than 50 countries, each with unique culture, landscape and tradition.

Africa’s diversity is apparent to anyone who travels to the continent. In the north, you have countries like Morocco, Algeria and Egypt, which have a long tradition of welcoming visitors. Further south there is Kenya and Tanzania, both renowned for their wildlife and for Africa’s tallest peak, Mt. Kilimanjaro. On the southern tip of the continent is South Africa, the region’s economic and political superpower which in itself lies many fascinating cultures. There are also the trouble spots like Congo and Sudan which are almost all but avoided by travelers.

Travel in Africa can be rough. It can also be a great joy. If you have patience, a willingness to maintain an open mind, a desire to explore and the ability to laugh at the strange, your African travels will change your life.

There are many entry points to Africa. Your best bet in terms of cheap airfare is to fly into one of the bigger cities like Johannesburg or Nairobi. Depending on where you are starting from flights to Africa can be either quite cheap or outrageously expensive. As is true nearly everywhere, flying into the biggest cities is usually your best bet for a cheap ticket.

Many people who travel to Africa prefer to do so in a group or through an organized adventure trip. This is especially the case with safari trips. BootsnAll has some of the best African adventure trips and safaris in the world. We work closely with local tour providers to offer you many options at very affordable rates.

If you are looking for a place to stay in, check out the hotels in Africa or the hostels in Africa for those on a tighter budget.

Still want more? You can learn from those who have been to Africa by sharingideas and thoughts on our Africa community message board.

1.From the first paragraph, we know when mentioning Africa, people often_______.

A.regard it as a continent, nota country    

B.connect it with poverty, disease and despair

C.consider it a nice place to travel in

D.admit there are more than 50 countries.

2.Africa’s tallest peak, Mt. Kilimanjaro is________.

A.in the north      B.in the south      C.in the middle    D.off the sea shore

3.What can we infer from the underlined part in Para.2?

A.Travelers seldom go to visit Congo or Sudan.

B.People in Congo and Sudan are trouble-makers.

C.Congo and Sudan are as powerful as South Africa.

D.There are few sightseeing spots in Congo or Sudan.

4.Your African travels will be a great joy, if you have_________.

①patience     ②a willingness to maintain an open mind

③a desire to explore     ④the ability to laugh at the strange

A.①②③     B.①③④     C.②③④     D.①②③④

5.What’s the author’s purpose in writing this passage?

A.Introducing some information of Africa.      

B.Encouraging people to travel in Africa.

C.Reminding travelers of the danger in Africa.

D.Telling people the cheap air fare to Africa.

填空题