a) Cách 1 : (3/4 + 1/2) x 5/7

               = 5/4 x 5/7

                = 25/28

Cách 2 : (3/4 + 1/2) x 5/7

           = 3/4 x 5/7 + 1/2 x 5/7

            = 15/28 + 5/14

            = 25/28

b) Cách 1 : 5/7 x 13/21 + 2/7 x 13/21

= 65/147 + 26/147

= 13/21

Cách 2 : 5/7 x 13/21 + 2/7 x 13/21

= (5/7 + 2/7) x 13/21

= 1 x 13/21

= 13/21

câu c tự làm

8 /15 :2/3+7/15:2/3 =                                              tính bằng 2 cách nhé các bạn

Cách 1:6/8+4/8+5/7=10/8*5/7=50/56=25/28

Cách 2 :3/4*5/7+1/2*
5/7=15/28+10/28=25/28

Cái này thầy tôi đã chữa yên tâm

7 x 2 = 14

4 x 7 = 28

7 x 6 = 42

3 x 7 = 21

5 x 7 = 35

2 x 7 = 14

7 x 4 = 28

6 x 7 = 42

7 x 3 = 21

7 x 5 = 35

7 x 2 = 14

4 x 7 = 28

7 x 6 = 42

3 x 7 = 21

5 x 7 = 35

2 x 7 = 14

7 x 4 = 28

6 x 7 = 42

7 x 3 = 21

7 x 5 = 35

`18/27-8/27`

`=10/27`

`4/7xx2/5`

`=8/35`

`3/4:2/7`

`=3/4xx7/2`

`=21/8`

a)

\(11\dfrac{3}{13}-\left(2\dfrac{4}{7}+5\dfrac{3}{13}\right)\)

\(=\dfrac{146}{13}-\left(\dfrac{18}{7}+\dfrac{68}{13}\right)\)

\(=\dfrac{146}{13}-\dfrac{18}{7}-\dfrac{68}{13}\)

\(=\left(\dfrac{146}{13}-\dfrac{68}{13}\right)-\dfrac{18}{7}\)

\(=6-\dfrac{18}{7}\)

\(=\dfrac{24}{7}\)

b)

\(\dfrac{2}{7}\times5\dfrac{1}{4}-\dfrac{2}{7}\times3\dfrac{1}{4}\)

\(=\dfrac{2}{7}\times\dfrac{21}{4}-\dfrac{2}{7}\times\dfrac{13}{4}\)

\(=\dfrac{2}{7}\times\left(\dfrac{21}{4}-\dfrac{13}{4}\right)\)

\(=\dfrac{2}{7}\times2\)

\(=\dfrac{4}{7}\)

\(a,11\dfrac{3}{13}-\left(2\dfrac{4}{7}+5\dfrac{3}{13}\right)\)

\(=\dfrac{146}{13}-\left(\dfrac{18}{7}+\dfrac{68}{13}\right)\)

\(=\dfrac{146}{13}-\dfrac{68}{13}-\dfrac{18}{7}\)

\(=6-\dfrac{18}{7}\)

\(=\dfrac{24}{7}\)

\(b,\dfrac{2}{7}\times5\dfrac{1}{4}-\dfrac{2}{7}\times3\dfrac{1}{4}\)

\(=\dfrac{2}{7}\times\dfrac{21}{4}-\dfrac{2}{7}\times\dfrac{13}{4}\)

\(=\dfrac{2}{7}\times\left(\dfrac{21}{4}-\dfrac{13}{4}\right)\)

\(=\dfrac{2}{7}\times2\)

\(=\dfrac{4}{7}\)

`9/2xx7/3-4/3xx9/2`

`=9/2xx(7/3-4/3)`

`=9/2xx3/3`

`=9/2xx1`

`=9/2`

Học sinh nhẩm và ghi kết quả như sau:

7 x 5 = 35

7 x 6 = 42

7 x 2 = 14

7 x 4 = 28

35 : 7 =5

42 : 7 = 6

14 : 7 = 2

28: 7 =4

35 : 5 = 7

42 : 6 = 7

14 : 2 = 7

28 : 4 = 7

Học sinh nhẩm và ghi kết quả như sau:

3 x 4 = 12    2 x 6 = 12     4 x 3 = 12    5 x 6 = 30

3 x 7 = 21    2 x 8 = 16     4 x 7 = 28    5 x 4 = 20

3 x 5 = 15   2 x 4 = 8        4 x 9 = 36    5 x 7 = 35

3 x 8 = 24    2 x 9 = 18     4 x 4 = 16    5 x 9 = 45

3 x4 = 12

3 x7 = 21

3 x5 = 15

3 x8 =24

2 x 6= 12

2 x8 = 16

2 x4 =8

2 x9 =18

4 x3 =12

4 x7 =28

4 x9 =36

4 x4 =16

5 x6 =30 

5 x4 =20

5 x7 =35

5 x9 =45

3 x 4 = 12    2 x 6 = 12     4 x 3 = 12    5 x 6 = 30

3 x 7 = 21    2 x 8 = 16     4 x 7 = 28    5 x 4 = 20

3 x 5 = 15   2 x 4 = 8        4 x 9 = 36    5 x 7 = 35

3 x 8 = 24    2 x 9 = 18     4 x 4 = 16    5 x 9 = 45

bằng 0 nhé

a: \(=\dfrac{4}{5}\cdot\dfrac{7}{3}\cdot\dfrac{3}{7}\cdot\dfrac{5}{4}+2011=2011+1=2012\)

b: \(=\dfrac{1}{3}\cdot\dfrac{9}{7}\cdot\dfrac{7}{9}=\dfrac{1}{3}\)

`a)4/5:3/7xx3/7:4/5+2011`

`=4/5xx7/3xx3/7xx5/4+2011`

`=(4/5xx5/4)xx(7/3xx3/7)+2011`

`=1xx1+2011`

`=1+2011`

`=2012`

__

`b)1/2xx2/3:7/9xx7/9`

`=1/2xx2/3xx9/7xx7/9`

`=(1/2xx2/3)xx(9/7xx7/9)`

`=1/3xx1`

`=1/3`