速算与巧算四年级奥数习题及答案

奥数对于开拓学生的思维能力,动脑能力有着非常大的帮助,小编准备了一些四年级奥数习题及答案,快来练习一下吧!

习题

1.计算899998+89998+8998+898+88

2.计算799999+79999+7999+799+79

3.计算(1988+1986+1984++6+4+2)-(1+3+5++1983+1985+1987)

4.计算12+34+56++19911992+1993

5.时钟1点钟敲1下，2点钟敲2下，3点钟敲3下，依次类推.从1点到12点这12个小时内时钟共敲了多少下?

6.求出从1～25的.全体自然数之和.

7.计算 1000+999998997+996+995994993++108+107106105+104+103102101

8.计算92+94+89+93+95+88+94+96+87

9.计算(12599+125)16

10.计算 3999+3+998+8+29+2+9

11.计算99999978053

12.两个10位数1111111111和9999999999的乘积中，有几个数字是奇数?

习题解答

1.利用凑整法解.

899998+89998+8998+898+88

=(899998+2)+(89998+2)+(8998+2)+(898+2)(88+2)-10

=900000+90000+9000+900+90-10

=999980.

2.利用凑整法解.

799999+79999+7999+799+79

=800000+80000+8000+800+80-5

=888875.

3.(1988+1986+1984++6+4+2)-(1+3+5++1983+1985+1987)

=1988+1986+1984++6+4+2-1-3-5

-1983-1985-1987

=(1988-1987)+(1986-1985)++(6-5)+(4-3)+(2-1)

=994.

4.1-2+34+5-6++1991-1992+1993=1+(3-2)+(5-4)++(1991-1990)+(1993-1992)

= 1+1996

=997.

5.1+2+3+4+5+6+7+8+9+10+11+12

=136=78(下).

6.1+2+3++24+25

=(1+25)+(2+24)+(3+23)++(11+15)+(12

+14)+13

=2612+13=325.

7.解法1：1000+999998997+996+995994-993++108+107106105+104+103102101=(1000+999998997)+(996+995994-993)++(108+107106105)+(104+103102101)

解法 2：原式=(1000998)+(999997)+(104102)

+(103101)

=2450

=900.

解法 3：原式=1000+(999998997+996)+(995994

-993+992)++(107106105+104)

+(103102101+100)-100

=1000100

=900.

9.(12599+125)16

=125(99+1)16

= 12510082

=12581002

=200000.

10.3999+3+998+8+29+2+9

= 3(999+1)+8(99+1)+2(9+1)+9

=31000+8100+210+9

=3829.

11.99999978053

=(10000001)78053

=7805300000078053

=78052921947.

12.11111111119999999999

=1111111111(100000000001)

=111111111100000000001111111111

=11111111108888888889.

这个积有10个数字是奇数.