加载中…
个人资料
  • 博客等级:
  • 博客积分:
  • 博客访问:
  • 关注人气:
  • 获赠金笔:0支
  • 赠出金笔:0支
  • 荣誉徽章:
正文 字体大小:

第一讲:加减法中的简便运算【三年级秋季班】

(2013-10-11 17:38:35)
标签:

加减法

简便运算

三年级

教育

分类: 三年级数学培训解答

知识导航

    1、简便运算的核心是凑整,凑整先算。加减法叫一级运算,乘除法叫二级运算。

2、在运算中,同级运算可以带符号搬家。要改变运算顺序可以加上或去掉括号。加号,乘号和等号后面加括号(或去括号),括号里面不变号;减号除号后面加括号(或去括号),括号里面要变号。

3、运用运算定律可以使计算简便,常用的运算定律有:

加法交换律:a+b=b+a;乘法交换律:a×b=b×a

加法结合律:(a+b)+c=a+(b+c);乘法结合律:(a×b)×c= a×(b×c)

乘法分配律:(a+b) ×c =a×c +b×c

  精典例题

1计算:  

思路点拨根据尾数凑整求和,19,28,37,64,55

=43+57+18+82+54

=100+100+54

=254

模仿练习

    用简便方法计算下面各题。

      1                   2

       =724+226+45+655                      =37+23+111+89+24

       =1000+700                                    =60+200+24

       =1700                                        =284

2

思路点拨 连减的性质:连续减去几个数,等于减去这几个数的和。

          =2000-53+40+60+47

          =2000-53+47+40+60

          =2000-200

          =1800

模仿练习

    用简便方法计算下面各题。

1                         2

         =213-86+114                               =2006-563+437+484+516

         =213-200                                      =2006-2000

         =13                                           =6

3

=1000-90+80+70+60+50+40+30+20+10

=1000-50×9

=1000-450

=550

3想一想,怎样计算更加简便。

1                             2

思路点拨      根据尾数“带着符号搬家”。

      =847-347+238                                       =651+149-385

      =500+238                                           =800-385

      =738                                               =415

 

模仿练习

    用简便方法计算。

1                            2

=456-256+376                                         =724+176-243

=200+376                                             =900-243

=576                                                 =657

 

4先观察,再动手计算。

1        2      3

思路点拨:利用去括号的法则,将数的“好朋友”从括号中解救出来。

      =643+257-186                     =3482-955-482                 =474-353+26

      =900-186                         =3482-482-955                 =474+26-353

      =714                             =3000-955                     =500-353

                                       =2045                         =147

模仿练习

    用简便方法计算。

    1          2          3

        =456-456+376                   =327-99+73                      =4253-253+158

        =376                           =327+73-99                      =4000+158

                                       =400-99                         =4158

                                       =301

5用简便方法巧算下面各题。

1      2      3       4

思路点拨多加了要减出去,少加的要继续加;多减了要加回来,少减的要继续减。

      =617-500+2              =512-300-4          =2000+35-1           =458+100+3

      =117+2                  =212-4              =2035-1              =558+3

      =119                    =208                =2034                =561

模仿练习

    1、用简便方法计算下面各题。

1       2     3       4

=300+87-2               =541+1000+3            =318-200+1            =1000-400-3

=387-2                  =1541+3                =118+1                =600-3

    =385                    =1544                  =119                  =597

 

2、怎样计算更简便?

1                        2

     =10+100+1000+10000-4                                =20+200+2000+20000-4

     =11110-4                                            =22220-4

     =11106                                              =22216

 

6计算: 。

思路点拨:利用基准数法,当一串数接近一个整数,可以都看作这个整数相加,再把多出的部分加上,少的部分减去,加减可以抵消。

                 =70×9-3-4+4+2-2-1+5+1

                 =630-3+5

                 =632

模仿练习

    用简便方法计算。

1      2

=100×8-1+1-2-3+2+3+3                               =2007×5

=800+3                                              =10035

=803

学以致用

A

1. 计算:

1      2    3

         =487+100-2             =748+1000+3          =42+58+71+29+24

         =587-2                 =1748+3              =100+100+24

         =585                   =1751                =224

2. 简便计算下列各题。        

   13007327    2100090802010 34723-(723189

         =300-(73+27)             =1000-(90+80+20+10)           =4723-723-189

        =300-200                  =1000-200                      =4000-189

        =100                      =800                           =3811

42356159356  53254612554    6536+(541464)+459 

=2356-356-159            =325-125+46+54               =536+541+464+459

=2000-159                =(325-125)+(46+54)           =(536+464)+(541+459)

=1841                    =200+100                     =1000+1000

                         =300                         =2000

71870280520              (84995-(995480

    =1870-(280+520)                               =4995-995+480

    =1870-800                                     =4000+480

    =1070                                         =4480

    3. 计算:(1      2 (华罗庚少年数学邀请赛集训试题)

=6211-200-2              =4796-2000+2

=6011-2                  =2796+2

              =6009                    =2798

4. 1      2       3

         =4356-356+1287                 =389+211-497             =1457-300-457

         =4000+1287                     =600-497                     =1157-457

         =5287                          =103                         =1000

B

5. 用简便方法计算。(湖北黄冈第三届小学生智力竞赛试题)。

1      2    3

     =568-(127+73)                =2008-(534+266)-208            =9000+900+90-3

     =568-200                     =2008-800-208                  =9990-3

           =368                         =1208-208                     =9987

                                        =1000

6. 你能在半分钟内计算出下面两题吗?试一试。

1       2

       =1350+49+68+51+32+1650                         =43+38+45+55+62+57

          =1350+1650+49+51+68+32             =43+57+38+62+45+55

          =3000+100+100                                  =100+100+100

          =3200                                          =300

7.

    =1000-(81+19+82+18+83+17+84+16+86+14+87+13+88+12+89+11)

    =1000-100×8

    =1000-800

=200

C

8. 给左边的算式找到好朋友,用线连起来。

                                    

                      

                     

                     

                      

9. 如图,用数字3从上到下叠罗汉,叠了10层,这10层的所有数字的和是多少?(2009年三年级希望杯竞赛训练试题)

 

3

3    3    3

3   3    3    3    3

3   3   3    3    3    3    3

……

观察:第一层有一个3,数字和就是3;

第二层有两个3,数字和是6;

第三层有三个3,数字和是9;

第四层有四个3,数字和是12;

…………

第十层有十个3,数字和是30;

所有数字的和是:3+6+9+……+12

               =15×5

               =75

答:所有数字的和是75.

 

0

阅读 收藏 喜欢 打印举报/Report
  

新浪BLOG意见反馈留言板 欢迎批评指正

新浪简介 | About Sina | 广告服务 | 联系我们 | 招聘信息 | 网站律师 | SINA English | 产品答疑

新浪公司 版权所有