《信号与系统》综合复习资料
一、简答题：
1、dt
t df t f x e t y t
)
()
()0()(+=-其中x(0)是初始状态，为全响应，为激励，)()(t y t f 试回答该系统是否是线性的？ 。
解答：由于无法区分零输入响应和零状态响应，因而系统为非线性的。 2、
4sin()()?6t t dt π
δ+∞
-∞-?=?__________________________。
解：根据冲激函数的性质：
2)60sin(4)()6sin(4--=-=-?∞
∞π
δπdt t t
3、()*(4)?k k εδ-=_答：)4(-k ε_____________________________________________。
4、已知系统的零状态响应和输入之间的关系为：()(1)zs y k f k =-，其中，激励为()f ?，
零状态响应为()zs y ?，试判断此系统是否是时不变的？ 。 解：设)()(0k k f k f -=，若系统为时不变的则有：
)1())(1()(000k k y k k y k k y zs zs zs +-=--=-
根据)1()(k f k y zs -=，则将题设代入，可得：)1()(0k k f k y zs --= 很明显，)()(0k k y k y zs zs -≠ 因而系统为时变的。
5、(2)t δ=___________________________。 答案：)(2
1
)2(t t δδ=
6、已知描述系统的微分方程为'()sin ()()y t ty t f t +=
其中()()f t y t 为激励，为响应，试判断此系统是否为时不变的？ 解：系统是时变的。 7、
()?t e t dt δ+∞
--∞
=?
____________________________。
解：1 。
8、已知信号3
()sin cos 62
f k k k π
π=+，则，该信号的周期为？ 解：设k k f 6
sin )(1π
=，其周期为121=T ；
设k k f 2
3sin
)(2π=，其周期为342=T ；
二者的最小公倍数为12，因而信号为周期信号，其周期为12=T .
9、线性是不变系统传输信号不失真的频域条件为：___________________________。 答：)()(0t t K t h -=δ
10、 设系统的激励为()f t ，系统的零状态响应)(t y zs 与激励之间的关系为：
)()(t f t y zs -=，判断该系统是否是线性的，并说明理由。
解：若系统为线性的，则应满足齐次性和可加性。 （1）齐次性。
设)()(1t af t f =，且)()(11t y t f zs =-若系统满足齐次性，必有：)()(1t ay t y zs zs =下面看结论是否成立。根据输入与输出之间的关系可得)()(t f t y zs -=，将题设代入可得到：
)()()()(11t ay t af t f t y zs zs =-=-=所以结论成立，从而系统满足齐次性。
（2）可加性。
设)()()(21t f t f t f +=, 其中，)()(11t y t f zs =-，)()(22t y t f zs =-，若系统满足可加性，则必有结论)()()(21t y t y t y zs zs +=。下面证明这一结论。
根据输入与输出之间的关系可得)()(t f t y zs -=，将题设代入可得到：
)()()()()]()([)()(212121t y t y t f t f t f t f t f t y zs zs zs +=-+-=-+-=-=
所以系统满足可加性。 综合（1）（2）可得，系统为线性的。
11、已知描述LTI 连续系统的框图如图所示，请写出描述系统的微分方程。
解：由于输入输入之间无直接联系，设中间变量)(t x 如图所示,则各积分器的的输入信号分别如图所示。由加法器的输入输出列些方程：
左边加法器：)(3)(2)()(t x t x t f t x '--='' （1） 右边加法器：)(2)()(t x t x t y '-''= （2） 由（1）式整理得到：)()(2)(3)(t f t x t x t x =+'+'' （3） 消去中间变量)(t x : )](2)([2)(2t x t x t y '-''= (4) )]'(2)([3)(3t x t x t y '-''=' (5)
])(2)([)('''-''=''t x t x t y (6)
将(4)(5)(6)左右两边同时相加可得：
)](2)([2])(2)([3])('2)([)(2)(3)(t x t x t x t x t x t x t y t y t y '-''+''-''+''-''=+'+''
整理可得到：
)(2)()(2)(3)(t f t f t y t y t y '-''=+'+''
12、已知一信号()f k 如图所示，请用单位阶跃序列()k ε及其移位序列表示()f k 。
答案： )4()1()(---=k k k f εε
1 ()f k
k
4 3 2 1 0
二、作图题：
1、已知信号()f k 的波形如图所示，画出信号(2)(2)f k k ε+?--的波形。
解：
2?
左移个单位
???
1
2
3
1
0 k
()k ε
1
???
-4 -3 -2 0 k
(2)k ε--
2?
右移个单位
1
???
2
3
4
0 k
(2)k ε-
?
翻转
再根据信号乘积，可以得到(2)(2)f k k ε+?--的波形：
2、已知
()()12f t f t 、的波形如下图，求()
()()12f t f t f t =*（可直接画出图形）
解：本题可以利用图解的方法，也可以利用卷积公式法来进行计算。
卷积公式法： 1()()(2)f t t t εε=--
2()()(1)f t t t εε=--
1212()()*()()()f t f t f t f f t d τττ+∞
-∞
==-?
12()()()[()(2)][()(1)]f t f f t d t t d τττετετετεττ+∞+∞-∞
-∞
=-=--?----?
?
()()()()(1)(2)()(2)(1)f t t d t d t d t d ετεττετεττ
ετεττετεττ
+∞
+∞-∞
-∞
+∞+∞
-∞
-∞
=-------+---????
利用阶跃函数的性质对上面的式子进行化简：
11
2
2
()()(1)(1)(2)(2)(3)(3)
t t t t f t d d d d t t t t t t t t ττττ
εεεε--=--+=------+--????
()[()(1)][(1)(2)](3)[(2)(3)]f t t t t t t t t t εεεεεε=--+--------
根据上面的表达式，可以画出图形：
3、已知信号()f t 的波形如图所示，画出信号1
(1)2
f t -
的波形。
解：
?
向左移动一个单位
?
翻转
4、已知信号)(t f 的波形如图所示，请画出函数)21(t f - 的波形。
解：
1
向左平移?
)
1(+t f ()
t f ?
横坐标展缩
翻转
?
/2
1压缩到原来的?
三、综合题目：（请写明步骤，否则不得分）
1、某LTI 系统的冲激响应()()2()h t t t δδ'=+，若激励信号为()f t 时，其零状态响应
()()t zs y t e t ε-=，求输入信号()f t 。
解：()()2()h t t t δδ'=+转换到s 域，可得：
2)(+=s s H
零状态响应为：()()t
zs y t e t ε-=，转换到s 域可得：
1
1
)(+=
s s Y zs ，则在s 域输入的象函数为： 2
111)2)(1(1211
)()()(+-+=++=++==s s s s s s s H s Y s F zs
取其拉氏反变换可得：
)()()(2t e e t f t t ε---=
2、某离散系统的输出()y k 与输入()f k 之间的关系为： 0
()2
()i
i y k f k i ∞
==
-∑
求系统的单位序列响应()h k 。 解：根据单位序列响应的概念可得：
∑∞
--=0
)(2)(i i i k k h δ
则：Λ+-+=)1(2)(2)(1
0k k k h δδ 观察规律可得：)(2)(k k h k
ε=
3、已知因果系统的差分方程为：()3(1)2(2)()y k y k y k f k +-+-=，其中，
()2()k f k k ε=。若已知(0)0,(1)2y y ==，求系统的全响应()y k 。
解：系统的齐次方程为：()3(1)2(2)0y k y k y k +-+-=
特征方程为：2
320λλ++= 所以特征根分别为：1212λλ=-=-， 所以系统的齐次解可以表示为：()1(1)2(2)k
k
yh k c c =-+-
已知系统的输入为()2()k
f k k ε=，则系统的特解可以表示为：()2k
yp k p =，将其代入到原差分方程，可得：13
p = 所以特解1()23
k
yp k =
所以系统的全解可表示为：
1
()()()1(1)2(2)23
k k k y k yh k yp k c c =+=-+-+
将初始条件(0)0,(1)2y y ==代入，可得待定系数：
2
1,213
c c ==-
所以系统的全响应为：21()(1)(2)(2),033
k k
k y k k =---+≥
4、图示离散系统有三个子系统组成，已知)4
cos(
2)(1π
k k h =， )()(2k a k h k ε=，激励)1()()(--=k a k k f δδ，求：零状态响应()zs y k 。
解：由题意可知，该系统为子系统的串联，则：
12()()*()h k h k h k =
()()*()zs y k f k h k =
所以12()()*[()*()]zs y k f k h k h k = 将已知条件代入有：
()[()(1)]*2cos(
)*()4
k zs k y k k a k a k π
δδε=-- 整理可得：
1()[()(1)]*()*2cos(
)4
[()(1)]*2cos()
4
k zs k k k y k k a k a k k a k aa k πδδεπ
εε-=--=--
[()(1)]*2cos(
)4
k k a k k π
εε=-- ()*2cos()4
k k a k πδ= ()*2cos(
)4
k k π
δ= 2cos(
)4
k π= 5、已知一个因果LTI 系统的输出()y t 与输入()f t 有下列微分方程来描述： ''()6'()8()2()y t f t y t f t ++= （1）确定系统的冲激响应()h t ；
（2）若2()()t
f t e t ε-=，求系统的零状态响应)(t y zs 。 解：（1）冲激响应()h t 满足方程
''()6'()8()2()h t h t h t t δ++=
及初始状态(0)(0)0h h --'== 对方程两边同时取拉氏变换：
2
()6()8()2s H s sH s H s ++= 整理得：2()211
()()6842
Y s H s F s s s s s -=
==+++++ 所以系统的冲激响应为： 24()()()t
t h t e e t ε--=-
（2）21
()()2
t
f t e
t s ε-=?
+ 零状态响应可以表示为：
222
()()()()
68
212
(2)(4)2(2)(4)
zs Y s H s F s F s s s s s s s s ==
++==
+++++
利用部分分式展开可得：
22
11
21
22()(2)(4)(2)24
zs Y s s s s s s ==-++++++ 取其逆变换可得： 所以 4211
()(())()2
2
t
t zs y t e t e t ε--=+-
6、已知某线性时不变系统对输入()f t 的零状态响应为：
()(1)t
t zs y t e f d τττ--∞
=-?，求该系统的单位冲激响应()h t 和频率响应()H j ω。
解： τττd f e t y t
t zs ?
∞
---=
)1()(
自变量的范围：t <<∞-τ即：0>-τt 该范围可用阶跃函数表示：)(τε-t 原方程可变为： ττεττd t f e t y t zs )()1()(--=
?
+∞
∞
--
利用单位冲激响应的定义可得：ττετδτd t e t h t )()1()(--=
?
+∞
∞
--
根据冲激函数的取样性质可得：)1()()(11-=-=-=-t e t e t h t t ετεττ 因为：1
1
)(+?
-ωεj t e t
所以利用时移特性有：ωωεj t
e j t e --+?
-1
1
)1(1 即：ωωωj e j j H -+=1
1
)(
7、已知某线性时不变连续系统的阶跃响应为3()(1.50.5)()t
t g t e
e t ε--=-；当系统的激励为
()(2)()f t t t ε=+，系统的初始值为(0)3,(0)9,y y ++'==-求系统的完全响应。
解：由于系统的阶跃响应为3()(1.50.5)()t
t g t e
e t ε--=-，根据阶跃响应与冲激响应)(t h 的
关系 可得：
)
(5.0)(5.4)()()5.05.4()()5.05.1()()(333t e t e t t e e t e e t g t h t t t t t t εεδεδ------+-=+-+-='=
将其转化到s 域，可得：4
315.035.41)(22
++=+++-=s s s s s s H 则描述系统的方程为：)()(3)(4)(t f t y t y t y ''=+'+'' 并将已知输入转化到s 域: 212)(s
s s F +=
则，系统的零状态响应的象函数为：)
3)(1(1
)3)(1(2)(2+++++=s s s s s s s Y zs
整理可得：3
1
251121)(++
+-
=s s s Y zs 取拉式反变换可得：3()(0.5 2.5)()t t
zs y t e e t ε--=-+
333()(0.57.5)()(0.5 2.5)() (0.57.5)()2()
t t t t
zs
t t
y t e e t e e t e e t t εδεδ------'=-+-+=-+
从而：(0)2,(0)5zs zs y y '+=+=- 所以：
(0)(0)(0)(0)321,
(0)(0)(0)(0)9(5)4
zi zi zs zi zi zs y y y y y y y y +=-=+-+=-=''''+=-=+-+=---=
因为描述系统的微分方程为：()4()3()()y t y t y t f t '''''++=
所以(0)(0)4(0)8 3.5 2.5()(1)(3)(1)(3)13
zi zi zi zi sy y y s Y s s s s s s s '-+-+-+-=
==+++++++
所以3()(3.5 2.5)()t t
zi y t e e t ε--=-
所以系统的全响应为：
()()()3()t zi zs y t y t y t e t ε-=+=
8、已知某LTI 连续系统的系统函数()2
31
22++++=s s s s s H ，求：
（1）系统的冲激响应()t h ；
（2）当激励)()(t t f ε=，初始状态()'
(0) 1 , 01y y --==时系统的零输入响应() zi y t 和
零状态响应()zs y t 。
解（1）因为)()(s H t h ?而2()()(3)()t
t
h t t e e
t δε--=+-
两边同时取拉普拉斯变换，可得：
)
2)(1()1(3)2()2)(1(23111)(+++-++++=+-++
=s s s s s s s s s H 整理可得：2
31
)2)(1()1(3)2()2)(1()(22++++=+++-++++=s s s s s s s s s s s H
（2）根据系统函数的定义：)()
()(s F s Y s H =而231)(22++++=s s s s s H
所以：231
)()(22++++=s s s s s F s Y ? )()1()()23(22s F s s s Y s s ++=++
两边同时取拉普拉斯逆变换，可得描述系统的微分方程为：
)()()()(2)(3)(t f t f t f t y t y t y +'+''=+'+''
而零输入响应)(t y zi 满足如下方程
0)(2)(3)('''=++t y t y t y zi zi zi
和初始状态：)0()0(--=y y zi )0()0('
--'=y y zi
对方程两边同时取拉普拉斯变换，可得：
0)(2_))0()((3_)0(_)0()(('
2=+-+--s Y y s sY y sy s Y s zi zi zi zi zi zi
整理可得：
2
3)
0(3)0()0()(2
'
++++=---s s y y sy s Y zi zi zi zi 将初始状态代入可得：1
3
22234)(2
+++-=+++=
s s s s s s Y zi 取拉普拉斯逆变换，可得系统的零输入响应为：)()32()(2t e e
t y t t
zi ε--+-=
)(*)()(t f t h t y zs =，所以：
)
2)(3(3)1)(3(13131)23111()()()(++-++++=++-++
==s s s s s s s s s F s H s Y zs
整理可得：
2
31213127332312132131)(+-
+++=+++-+++-++=s s s s s s s s s Y zs 取拉普拉斯逆变换可得系统的零状态响应为：)()2
7
32
1()(32t e e e t y t t t
zs ε---+-=
2018年12月大学英语四级考试真题（第1套） Part I Writing (30minutes) Directions: For this part, you are allowed 30 minutes to write a short essay on the challenges of living in a big city. You should write at least120 words but no more than 180 words. ___________________________________________________________________________________________________ ___________________________________________________________________________________________________ ___________________________________________________________________________________________________ Part II Listening Comprehension (25 minutes) Section A Directions: In this section, you will hear three news reports. At the end of each news report, you will hear two or three questions. Both the news report and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A), B), C) and D). Then mark the corresponding letter on Answer Sheet 1 with a single line through the centre. Questions 1 and 2 are based on the news report you have just heard. 1. A) Land a space vehicle on the moon in 2019. B) Design a new generation of mobile phones. C) Set up a mobile phone network on the moon. D) Gather data from the moon with a tiny device. 2. A) It is stable.B) It is durable. C) It is inexpensive.D) It is sophisticated. Questions 3 and 4 are based on the news report you have just heard. 3. A) It lasted more than six hours. B) No injuries were yet reported. C) Nobody was in the building when it broke out. D) It had burned for 45 minutes by the time firefighters arrived. 4. A) Recruit and train more firefighters. B) Pull down the deserted shopping mall. C) Turn the shopping mall into an amusement park. D) Find money to renovate the local neighborhood. Questions 5 to 7 are based on the news report you have just heard. 5. A) Shrinking potato farming.B) Heavy reliance on import. C) Widespread plant disease.D) Insufficient potato supply. 6. A) It intends to keep its traditional diet. B) It wants to expand its own farming. C) It is afraid of the spread of disease. D) It is worried about unfair competition. 7. A) Global warming.B) Ever-rising prices. C) Government regulation.D) Diminishing investment. Section B Directions：In this section, you will hear two long conversations. At the end of each conversation, you will hear four questions. Both the conversation and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A), B), C), and D). Then mark the corresponding letter on Answer Sheet 1
Part I Writing (30 minutes) Directions:For this part, you are allowed 30 minutes to write an a short easy on the importance of writing ability and how to develop it.You should write at least 120 words but no more than 180 words. Part ⅢReading Comprehension (40 minutes) Section A Directions: In this section, there is a passage with ten blanks. You are required to select one word for each blank from a list of choices given in a word bank following the passage. Read the passage through carefully before making your choices, Each choice in the bank is identified by a letter. Please mark the corresponding letter for each item on Answer Sheet 2 with a single line through the centre. You may not use any of the words in the bank more than once. Questions 26 to 35 are based on the following passage. Since the 1940s, southern California has had a reputation for smog. Things are not as bad as they once were but, according to the American Lung Association, Los Angeles is still the worst city in the United States for levels of (26)_______ Gazing down on the city from the Getty Center, an art museum in the Santa Monica Mountains, one would find the view of the Pacific Ocean blurred by the haze (霾）.Nor is the state's bad air (27)_______ to its south. Fresno, in the central valley, comes top of the list in America for year-round pollution. Residents' hearts and lungs are affected as a (28)_______. All of which, combined with California 's reputation as the home of technological
2018年6月大学英语四级真题试卷一及答案(完整版) Part I Writing (30 minutes) (请于正式开考后半小时内完成该部分，之后将进行听力考试) Directions:For this part, you are allowed 30 minutes to write an a short easy on the importance of speaking ability and how to develop it.You should write at least 120 words but no more than 180 words. Part II Listening Comprehension (25 minutes) Section A Directions:In this section, you will hear three news reports. At the end of each news report, you will hear two or three questions. Both the news report and questions will be spoken only once. After you hear questions, you must choose the best answer from the four choices marked A), B), C) and D). Then mark the corresponding letter on Answer Sheet 1 with a single line through the centre. 注意：此部分试题请在答题卡1上作答。 Questions 1 to 2 are based on the new report you have just heard. 1. A) The return of a bottled message to its owner's daughter. B)A New Hampshire man's joke with friends on his wife. C)A father's message for his daughter. D)The history of a century-old motel. 2. A) She wanted to show gratitude for his kindness. B)She wanted to honor her father's promise.
2019年6月大学英语四级真题完整版（第3套） Part I Writing (30 minutes) Directions: For this part, you are allowed 30 minutes to write a news report to your campus newspaper on a visit to a volunteer activity organized by your Student Union. You should write at least 120 words but no more than 180 words. PartⅡListening Comprehension (25 minutes) Section A Directions: In this section, you will hear three news reports. At the end of each news report, you will hear two or three questions. Both the news report and then questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A），B), C) and D).Then mark the corresponding letter on Answer Sheet 1with a single line through the centre. 本次四级考试只有2套听力 Part ⅢReading Comprehension ( 40 minutes) Section A Directions:In this section, there is a passage with ten blanks. You are required to select one word for each blank from a list of choices given in a word bank following the passage. Read the passage through carefully before making your choices. Each choice in the bank is identified by a letter. Please mark the corresponding letter for each item on Answer Sheet 2 with a single line through the centre. You may not use any of the words in the bank more than once． Questions 26 to 35 are based on the following passage. Just because they can’t sing opera or ride a bicycle doesn’t mean that animals don’t have culture. There’s no better example of this than killer whales. As one of the most __26__ predators(食肉动物)，killer whales may not fit the __27__ of a cultured creature. However, these beasts of the sea do display a vast range of highly __28__ behaviors that appear to be driving their genetic development. The word “culture”comes from the Latin “colere,”which __29__ means “to cultivate.”In other words, it refers to anything that is __30__ or learnt, rather than instinctive or natural. Among human populations, culture not only affects the way we live, but also writes itself into our genes, affecting who we are. For instance, having spent many generations hunting the fat marine mammals of the Arctic, the Eskimos of
历年英语四级真题作文范文汇总 2010年12月大学英语四级作文真题 Directions：For this part, you are allowed 30 minutes to write a short essay entitled How Should Parents Help Children to Be Independent? You should write at least 150 words following the outline given below. 1. 目前不少父母为孩子包办一切 2. 为了让孩子独立, 父母应该…… How Should Parents Help Children to Be Independent? 参考范文 Help Children to Be Independent Thanks to the “family plan”policy,today in China many families have one child.Loving and caring for children is an old Chinese tradition that has remained for thousands of years.But the children are spoilt so much that they have less independence.They rely on their parents in everything.That is bad for a child's growing.Therefore,parents should develop the children's independence instead of doing everything for them. There are some ways to help children to be independent.First of all,you should believe your children can do the things well.Second,give them more chances to practise,when they don't know how to do it,just tell them the way.Third,don't be afraid to see them fail.In the beginning,they may do something bad.But don't worry,just let them try again.They can do the work as well as you if given more time. To be independent is good for the children.A child who is independent will succeed easily in the future. 2010年12月大学英语四级作文真题 Due Attention Should Be Given to Spelling 1.如今不少学生在英语(论坛)学习中不重视拼写 2.出现这种情况的原因 3.这种现象导致的结果 范文参考： Due Attention Should Be Given to Spelling
2014年6月大学英语四级考试真题及答案(完整版) Part I Writing (30 minutes) Directions:For this part, you are allowed30 minutes to write a short essay on the following topic. You should write atleast 120 words but no more than 180 words. Supposea foreign friend of yours is coming to visit your hometown, what is the mostinteresting place you would like to take him/her to see and why 注意：此部分试题请在答题卡1上作答。 PartII Listening Comprehension (30 minutes) SectionA Directions:In this section, you will hear 8 shortconversations and 2 long conversations. At the end of each conversation, one ormore questions will be asked about what was said. Both the conversation and thequestions will be spoken only once. After each question there will be a the pause, you must read the four choices marked A), B), C) and D), anddecide which is the best answer. Then mark the corresponding letter on Answer Sheet 1 with a single linethrough the centre. 注意：此部分试题请在答题卡1上作答。 See a doctor about her strained shoulder. B) Usea ladder to help her reach the tea. C)Replace the cupboard with a new one. D)Place the tea on a lower shelf next time. At Mary Johnson’s. C) In an exhibition hall.
历年英语四级考试真题及答案 （2007.6—2012.6 共十一套） 2007年6月英语四级真题及答案 Part I Writing (30 minutes) 注意：此部分试题在答题卡1上。 Directions: For this part, you are allowed 30 minutes to write a short essay on the topic of Welcom to our club. You should write at least 120 words following the outline given bellow: 欢迎辞，欢迎加入俱乐部。 标题：Welcome to our club 书写提纲： 1. 表达你的欢迎； 2. 对你们俱乐部作一个简要介绍。 Part II Reading Comprehension (Skimming and Scanning) (15 minutes) Directions: In this part, you will have 15 minutes to go over the passage quickly and answer the question on Answer Sheet 1. For questions 1-7, markY (for YES) if the statement agrees with the information given in the passage;N (for NO) if statement contradicts the information given in the passage;NG (for NOT GIVEN) if the information is not given in the passage.For question 8-10, complete the sentences with the information given in the passage. Protect Your Privacy When Job-hunting Online Identity theft and identity fraud are terms used to refer to all types of crime in which someone wrongfully obtains and uses another person’s personal data in some way that involves fraud or deception typically for economic gain. The numbers associated with identity theft are beginning to add up fast these days. A recent Genera Accounting Office report estimates that as many as 750,000 Americans are victims of identity theft ever
2017年英语四级考试真题及答案 Part I Writi ng (25 minu tes) Directions: For this part, you are allowed 30 minu tes to write a short easy on how to bestha ndle the relati on ship betwee n doctors and patie nts. You should write at least120 words but no more tha n 180 words. Part II Liste ning Comprehe nsion (25 minu tes) Sectio n A Directions: In this secti on, you will hear three n ews reports. At the end of each n ews report, you will hear two or three questions. Both the news report and the questions will be spoken only on ce. After you hear a questi on, you must choose the best an swer from the four choices marked A) , B), C) and D). Then mark the corresp onding letter on An swer Sheet 1 with a sin gle line through the cen tre. Questi ons 1 and 2 are based on the n ews report you have just heard. 1. A) Her friend Erika. C) Her grandfather. B) Her little brother. D) Her gran dmother. 2. A) By tak ing pictures for passers-by. C) By selli ng lem on ade and pictures. B) By work ing part time at a hospital. D) By ask ing for help on social media. Questi ons 3 and 4 are based on the n ews report you have just heard. 3. A) Finding cheaper ways of highway con structi on. B) Gen erati ng electric power for pass ing vehicles. C) Providi ng clea n en ergy to five millio n people. D) Testi ng the efficie ncy of the new solar pan el. 4. A) They can sta nd the wear and tear of n atural eleme nts. B) They can be laid right on top of existing highways. C) They are only about half an inch thick. D) They are made from cheap materials. Questi ons 5 to 7 are based on the n ews report you have just heard. 5. A) En dless fight ing in the regi on. C) In adequate funding for research. B) The hazards from the desert. D) The lack of clues about the species. 6. A) To observe the wildlife in the two national parks.
. . 英语四级阅读题库含答案解析 1.Passage One Global warming may or may not be the great environmental crisis of the 21st century, but regardless of whether it is or isn’t –we won’t do much about it. We will argue over it and may even, as a nation, make some fairly solemn-sounding commitments to avoid it. But the more dramatic and meaningful these commitments seem, the less likely they are to be observed. Al Gore calls global warming an “inconvenient truth,”as if merely recognizing it could put us on a path to a solution. But the real truth is that we don’t know enough to relieve global warming, and –without major technological breakthroughs—we can’t do much about it. From 2003 to 2050, the world’s population is projected to grow from 6.4 billion to 9.1 billion, a 42% increase. If energy use per person and technology remain the same, total energy use and greenhouse gas emissions (mainly, CO2) will be 42% higher in 2050. but that’s too low, because societies that grow richer use more energy. We need economic growth unless we condemn the world’s poor to their present poverty and freeze everyone else‘s living standards. With modest growth, energy use and greenhouse emissions more than double by 2050. No government will adopt rigid restrictions on economic growth and personal freedom (limits on electricity usage, driving and travel) that might cut back global warming. Still, politicians want to show they’re “doing something.”Consider the Kyoto Protocol (京都议定书). It allowed countries that joined to punish those that didn’t. But it hasn’t reduced CO2 emissions (up about 25% since 1990), and many signatories (签字国) didn’t adopt tough enough policies to hit their 2008-2012 targets. The practical conclusion is that if global warming is a potential disaster, the only solution is new technology. Only an aggressive research and development program might find ways of breaking dependence on fossil fuels or dealing with it. The trouble with the global warming debate is that it has become a moral problem when it’s really an engineering one. The inconvenient truth is that if we don’t solve the engineering problem, we’re helpless. 57. What is said about global warming in the first paragraph? A) It may not prove an environmental crisis at all. B) It is an issue requiring world wide commitments. C) Serious steps have been taken to avoid or stop it. D) Very little will be done to bring it under control. 58. According to the author’s understanding, what is Al Gore’s view on global warming? A) It is a reality both people and politicians are unaware of. B) It is a phenomenon that causes us many inconveniences. C) It is a problem that can be solved once it is recognized. D) It is an area we actually have little knowledge about. 59. Green house emissions will more than double by 2050 because of _______. A) economic growth B) the widening gap between the rich and poor C) wasteful use of energy D) the rapid advances of science and technology 60. The author believes that, since the signing of the Kyoto Protocol, ________.
2017年6月大学英语四级真题及答案 (第一套) Part I Writing (25 minutes) (请于正式开考后半小时内完成该部分，之后将进行听力考试) Directions:For this part, you are allowed 30 minutes to write an advertisement on your campus website to sell a computer you used at college. Your advertisement may include its brand, specifications/features, condition and price, and your contact information.You should write at least 120 words but no more than 180 words. Part II Listening Comprehension (30 minutes) Section A Directions:In this section, you will hear three news reports. At the end of each news report, you will hear two or three questions. Both the news report and questions will be spoken only once. After you hear questions, you must choose the best answer from the four choices marked A), B), C) and D). Then mark the corresponding letter on Answer Sheet 1 with a single line through the centre. 注意：此部分试题请在答题卡1上作答。 Questions 1 to 2 are based on the conversation you have just heard. 1.A) The man in the car was absent-minded. B) The test driver made a wrong judgement. C) The self-driving system was faulty. D) The car was moving at a fast speed. 2. A) They have done better than conventional cars. B) They have caused several severe crashes. C) They have posed a threat to other drivers. D) They have generally done quite well.
2016年6月四级考试真题(第三套) Part Ⅰ Writing (30 minutes) Directions：For this part,you are allowed 30 minutes to write a letter to express your thanks to one of your school teachers upon entering college.You should write at least 120 words but no more than 180 words． Part Ⅱ Listening Comprehension (25 minutes) 说明：2016年6月四级真题全国共考了两套听力。本套（即第三套）的听力内容与第二套的完全一样，只是选项的顺序不一样而已，故在本套中不再重复给出。 Part Ⅲ Reading Comprehension (40 minutes) Section A Directions：In this section，there is a passage with ten blanks．You are required to select one word for each blank from a list of choices given in a word bank following the passage．Read the passage through carefully before making your choices．Each choice in the bank is identified by a letter． Please mark the corresponding letter for each item on Answer Sheet 2 with a single line through the centre．You may not use any of the words in the bank more than once． Physical activity does the body good，and there’s growing evidence that it helps the brain too．Researchers in the Netherlands report that children who get more exercise，whether at school or on their own．26 to have higher GPAs and better scores on standardized tests．In a 27 of 14 studies that looked at physical activity and academic 28，investigators found that the more children moved．the better their grades were in school，29 in the basic subjects of math，English and reading． The data will certainly fuel the ongoing debate over whether physical education classes should be cut as schools struggle to 30 on smaller budgets．The arguments against physical education have included concerns that gym time may be taking away from study time．With standardized test scores in the U．S．31 in recent years，some administrators believe students need to spend more time in the classroom instead of on the playground．But as these findings show,exercise and academics may not be 32 exclusive．Physical activity can improve blood 33 to the brain，fueling memory,attention and creativity,which are34 to learning．And exercise releases hormones that can improve 35 and relieve stress，which can also help learning．So while it may seem as if kids are just exercising their bodies when they，re running around，they may actually be exercising their brains as well． A．attendance B．consequently C．current D．depressing E．dropping F．essential G．feasible H．flow I．mood J．mutually K．particularly L．performance M．review N．survive O．tend Section B Directions：In this section，you are going to read a passage with ten statements attached to it．Each statement contains information given in one of the paragraphs．Identify the paragraph from which the information is derived You may choose a paragraph more than once．Each paragraph is marked with a letter．Answer the questions by marking the corresponding letter on Answer Sheet 2． Finding the Right Home—and Contentment，Too