小学四年级奥数教程-速算与巧算(一)
速算与巧算(一)
计算是数学的基础,小学生要学好数学,必须具有过硬的计算本领。准确、快速的计算能力既是一种技巧,也是一种思维训练,既能提高计算效率、节省计算时间,更可以锻炼记忆力,提高分析、判断能力,促进思维和智力的发展。
我们在三年级已经讲过一些四则运算的速算与巧算的方法,本讲和下一讲主要介绍加法的基准数法和乘法的补同与同补速算法。
例1 四年级一班第一小组有10名同学,某次数学测验的成绩(分数)如下:
86,78,77,83,91,74,92,69,84,75。
求这10名同学的总分。
分析与解:通常的做法是将这10个数直接相加,但这些数杂乱无章,直接相加既繁且易错。观察这些数不难发现,这些数虽然大小不等,但相差不大。我们可以选择一个适当的数作“基准”,比如以“80”作基准,这10个数与80的差如下:
6,-2,-3,3,11,-6,12,-11,4,-5,其中“-”号表示这个数比80小。于是得到
总和=80×10+(6-2-3+3+11-
=800+9=809。
实际计算时只需口算,将这些数与80的差逐一累加。为了清楚起见,将这一过程表示如下:
通过口算,得到差数累加为9,再加上80×10,就可口算出结果为809。
例1所用的方法叫做加法的基准数法。这种方法适用于加数较多,而且所有的加数相差不大的情况。作为“基准”的数(如例1的80)叫做基准数,各数与基准数的差的和叫做累计差。由例1得到:
总和数=基准数×加数的个数+累计差,
平均数=基准数+累计差÷加数的个数。
在使用基准数法时,应选取与各数的差较小的数作为基准数,这样才容易计算累计差。同时考虑到基准数与加数个数的乘法能够方便地计算出来,所以基准数应尽量选取整
十、整百的数。
例2 某农场有10块麦田,每块的产量如下(单位:千克):
462,480,443,420,473,429,468,439,475,461。求平均每块麦田的产量。
解:选基准数为450,则
累计差=12+30-7-30+23-21+18-11+25+11
=50,
平均每块产量=450+50÷10=455(千克)。
答:平均每块麦田的产量为455千克。
求一位数的平方,在乘法口诀的九九表中已经被同学们熟知,如7×7=49(七七四十九)。对于两位数的平方,大多数同学只是背熟了10~20的平方,而21~99的平方就不大熟悉了。有没有什么窍门,能够迅速算出两位数的平方呢?这里向同学们介绍一种方法——凑整补零法。所谓凑整补零法,就是用所求数与最接近的整十数的差,通过移多补少,将所求数转化成一个整十数乘以另一数,再加上零头的平方数。下面通过例题来说明这一方法。
例3 求292和822的值。
解:292=29×29
=(29+1)×(29-1)+12
=30×28+1
=840+1
=841。
822=82×82
=(82-2)×(82+2)+22
=80×84+4
=6720+4
=6724。
由上例看出,因为29比30少1,所以给29“补”1,这叫“补少”;因为82比80多2,所以从82中“移走”2,这叫“移多”。因为是两个相同数相乘,所以对其中一个数“移多补少”后,还需要在另一个数上“找齐”。本例中,给一个29补1,就要给另一个29减1;给一个82减了2,就要给另一个82加上2。最后,还要加上“移多补少”的数的平方。
由凑整补零法计算352,得
35×35=40×30+52=1225。这与三年级学的个位数是5的数的平方的速算方法结果相同。
这种方法不仅适用于求两位数的平方值,也适用于求三位数或更多位数的平方值。 例4 求9932和20042的值。
解:9932=993×993
=(993+7)×(993-7)+72
=1000×986+49
=986000+49
=986049。
20042=2004×2004
=(2004-4)×(2004+4)+42
=2000×2008+16
=4016000+16
=4016016。
下面,我们介绍一类特殊情况的乘法的速算方法。
请看下面的算式:
66×46,73×88,19×44。
这几道算式具有一个共同特点,两个因数都是两位数,一个因数的十位数与个位数相同,另一因数的十位数与个位数之和为10。这类算式有非常简便的速算方法。
例5 88×64=?
分析与解:由乘法分配律和结合律,得到
88×64
=(80+8)×(60+4)
=(80+8)×60+(80+8)×4
=80×60+8×60+80×4+8×4
=80×60+80×6+80×4+8×4
=80×(60+6+4)+8×4
=80×(60+10)+8×4
=8×(6+1)×100+8×4。
于是,我们得到下面的速算式:
由上式看出,积的末两位数是两个因数的个位数之积,本例为8×4;积中从百位起前面的数是“个位与十位相同的因数”的十位数与“个位与十位之和为10的因数”的十位数加1的乘积,本例为8×(6+1)。
例6 77×91=?
解:由例3的解法得到
由上式看出,当两个因数的个位数之积是一位数时,应在十位上补一个0,本例为7×1=07。
用这种速算法只需口算就可以方便地解答出这类两位数的乘法计算。
练习1
1.求下面10个数的总和:
165,152,168,171,148,156,169,161,157,149。
2.农业科研小组测定麦苗的生长情况,量出12株麦苗的高度分别为(单位:厘米): 26,25,25,23,27,28,26,24,29,27,27,25。求这批麦苗的平均高度。
3.某车间有9个工人加工零件,他们加工零件的个数分别为:
68,91,84,75,78,81,83,72,79。
他们共加工了多少个零件?
4.计算:
13+16+10+11+17+12+15+12+16+13+12。
5.计算下列各题:
(1)372; (2)532; (3)912;
(4)682: (5)1082; (6)3972。
6.计算下列各题:
(1)77×28;(2)66×55;
(3)33×19;(4)82×44;
(5)37×33;(6)46×99。
练习1 答案
1.1596。 2.26厘米。
3.711个。 4.147。
5.(1)1369; (2)2809; (3)8281;
(4)4624; (5)11664; (6)157609。
6.(1)2156; (2)3630; (3)627;
(4)3608; (5)1221; (6)4554。
Class: Name:
( )1. Good morning! !
A.Morning ! B.Hello ! C.Hi !
( )2. Nice to see you again ! .
A.How are you ? B.Nice to see you , too . C.How do you do ?
( )3.Good night,mom !
A.Night ! B.Good night ! C.Good evening .
( )4.How do you do ?
A.How are you ? B.Fine,thanks . C.How do you do ?
( )5.How many story books do you have ?
A.I have 10. B.I can see 10. C.Thirty yuan.
( )6.Do you have new teachers?
A.Yes,we do . B.Yes,we don’t. C.Yes,we have .
( )7.Who’s your art teacher ?
A.Mr Zhu. B.Miss Zhu. C.He’s tall.
( )8.What’s he like?
A.He’s tall and strong . B.Yes,he is. C.Mr Zhu.
( )9.Is your English teacher young?
A.No,she isn’t. B.Yes,she is . C.No,she is.
( )10. ? Her name is Chen Jie.
A.What’s your name ? B.What’s she name ? C.What’s her name ?
( )11. ? I like Chinese,math and English.
A.What classes do you like? B.What do you like? C.What are you like ?
( )12. ? We have English and P.E.
A.What do you have on Mondays ? B.What do you have ?
C.What do you have on Monday?
( )13. ? It’s Monday.
A.What is it today ? B.What day is it today ? C.What day is today ?
( )14. ? I watch TV and do my homework.
A.What do you do ? B.What do you do in Mondays?
C.What do you do on Sundays ?
( )15.May I have a look ?
A.Sure.Here you are . B.Look ! C.Here you are .
( )16.Our math teacher is Canada.
A.from B.in C.at
( )17.I three new teachers.
A.has B.am C.have
( )18.What’s Chinese teacher like ?
A.you B.your C.you’re
( )19.My P.E.teacher is thin.
A.too B.so C.very
( )20.There are days in a week.
A.six B.seven C.eight
( )21.There are month(月)in a year(年).
A.ten B.eleven C.twelve
( )22.I often watch TV Saturday .
A.on B.in C.at
( )23.I like P.E. I don’t lime music.
A.but B.and C.so
( )24.This is apple. It is red apple.
A.a , a B.an,an C.an, a
( )25.What do you like ?
A.classes B.class C.class’s
六年级英语测试题
Class: Name:
( )1. How are you ?
A.Fine,thanks. B.Yes,it is. C.How are you ?
( )2. Nice to meet you !
A.Fine,thank you. B.OK. C.Nice to meet you ,too !
( )3. How do you go to school ?
A.I go to Canada by plane. B.I go to school by bike. C.What about you ?
( )4. How do you go to the USA ?
A.I usually go to school by bus. B.I go to England by ship. C.I go by plane.
( )5. ? My home is near the post office.
A. Where is your home ? B.OK. C.See you then!
( )6. See you at 2 o’clock.
A.See you then ! B.The fifth floor. C.It’s easy.
( )7. ? You can go by the No.15 bus.
A.It’s not far. B. How can I get to Zhongshan Park ? C.Sure.
( )8. Where is the hospital?
A.Next to the cinema. B.Thank you . C.You’re welcome.
( )9. Excuse me ,is there a cinema near here ?
A.Yes,there is . B.No,it’s not far. C.It’s near the post office.
( )10.? ? It’s near the post office.
A.Yes,there is . B.No,it’s not far. C. Where is the library?
( )11. ? It’s next to the hospital.
A. Where is the cinema ,please ? B.Is it far ? C.Go straight.
( )12. How can I get to the hospital ?
A.It’s next to the hospital. B.You can go by the No.201 bus. C.Thank you.
( )13.Thank you .
A.Thank you . B.OK. C.You’re welcome.
( )14. How can I get to the museum?
A.Go straight.Then turn left. B.Thank you . C.It’s east of the cinema .
( )15. Where is the post office ?
A.Go straight.Then turn left. B.Thank you . C.It’s east of the cinema .
( )16. What are you going to do this evening ?
A.I’m going to the cinema . B.I go to school on foot. C.Yes,it is.
( )17. Is it far ?
A.No,it is . B.Yes , it is . C.Yes,it isn’t.
( )18. do you go to school ?
A.What B.Where C.How
( )19. I go to school bike.
A.on B. by C.get
( )20. Can I go foot ?
A.by B.at C.on
( )21.How can I to the post office ?
A.near B. get C.for
( )22. me.
A. Excuse B. How C.next
( )23. birthday to you !
A. After B.Happy C.First
( )24. The hospital is the left.
A. at B.in C.on
( )25. is the bookstore ?
A.Where B.How C.When
四年级英语测试题
Class: Name:
( )1.Good afternoon !
A.Hello! B.Hi ! C.Afternoon !
( )2.Nice to see you again !
A.How are you ? B.Hello ! C.Nice to see you , too.
( )3.Good night,moom !
A.Good evening ! B.Good night ! C.Night !
( )4.How are you ?
A.Fine,thank you . B.I am 10. C.Nine .
( )5.How do you do ?
A.Fine ,Thank you . B.How are you ? C.How do you do ?
( )6.How old are you ?
A.How are you ? B.I am 11. C.I have 11.
( )7. ? I have 23.
A.How many books do you have ? B.How many books can you see?
C.How many book do you have ?
( )8. ? I can see 6.
A.How many lights do you have ? B.How many lights can you see ?
C.How many light can you see ?
( )9.May I have a look ?
A.Sure.Here you are . B.Look! C.Sure.Here are you .
( )10. ? 50 yuan.
A.How much is this schoolbag ? B.How many is this schoolbag ?
C.How much are this schoolbag ?
( )11.Where’s my seat ?
A.It’s near the door. B.It’s on the door. C.It’s under the door.
( )12.Let’s clean the desks and chairs.
A.All right ! B.Hello ! C.It’s nice .
( )13.What’s this ?
A.There is a board . B.It’s a bee. C.It’s bee.
( )14. ? My name is Mike.
A.Here are you ? B.What’s your name ? C.How do you do ?
( )15.Who’s the inventor of paper ?
A.Chinese people. B.Oh! Great ! C.Hello!
( )16.I a student. You a teacher.
A.am ,am B.are, are C.am, are
( )17.This a boy.His name Zhang Peng.
A.is, is B.am , is C.is , are
( )18.Let clean the fish bowl.
A.I B. me C.my
( )19.This is apple.It is red apple .
A.an ,an B.a, a C.an, a
( )20.We a new classroom.
A.are B.have C.has
( )21.There a bee in our classroom.
A. is B.are C.am
( )22. have a new schoolbag. schoolbag is heavy.
A.I , my B.I ,My C.My , I
( )23.Put your English book your head.
A.at B.in C.on
( )24.There many books in the desk.
A.am B.is C.are
( )25.How many do you have ? A.pencil B.pencils C.pencils. 三年级英语测试题
Class: Name:
( )1.当向别人打招呼时,应该说:
A.Hello. B.Good morning .
( )2.How are you ? 的正确答语是:
A.name B.I’m fine,thank you .
( )3.当想知道别人的名字时,应该说:
A.What’s your name ? B.See you.
( )4.字母K的小写是:
A.k B.
( )5.Nice to meet you 的意思是:
A.见到你很高兴。 B.你好吗?
( )6.向别人告别时,应该说:
A.Good bye! B.Hi.
( )7.Jenny is a
A.boy B.girl
( )8.Li Ming is a
A.boy B.girl
( )9.Jenny lives in
A.Canada. B.China.
( )10.字母P的大写是:
A.P B.q
( )11.当向别人说谢谢时,应该说:
A.Nice to meet you . B.See you later.
( )12.早上好的正确答语是:
A.Good morning . B.See you later.’
( )13.What’s this ? It’s a
A.desk B.book
( )14.This is a
A.teacher B.Danny
( )15.This is a boy.What’s name ?
A.his B.her