\(\dfrac{1}{7}\times\dfrac{21}{8}-\dfrac{3}{8}\times\dfrac{1}{7}-\dfrac{1}{7}\times\dfrac{2}{8}\\ =\dfrac{1}{7}\times\left(\dfrac{21}{8}-\dfrac{3}{8}-\dfrac{2}{8}\right)\\ =\dfrac{1}{7}\times\dfrac{16}{8}\\ =\dfrac{1}{7}\times2\\ =\dfrac{2}{7}\)

\(\dfrac{1}{7}\times\dfrac{21}{8}-\dfrac{3}{8}\times\dfrac{1}{7}-\dfrac{1}{7}\times\dfrac{2}{8}\)

=\(\dfrac{1}{7}\times\left(\dfrac{21}{8}-\dfrac{3}{8}-\dfrac{2}{8}\right)\)

=\(\dfrac{1}{7}\times2\)

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

`1/3 + 5/4`

`= 4/12 + 15/12`

`= 19/12

__

`4/12 - 1/15`

`= 1/3 -1/15`

`= 5/15 -1/15`

`=4/15`

__

`2/3 xx 15/8`

`= (2xx15)/(3xx88)`

`= 30/24`

`=5/4`

__

`2/3 : 8/21`

`= 2/3 xx 21/8`

`= (2xx21)/(3xx8)`

`= 42/24`

`=7/4`

1/3+5/4=4/12+15/12

=19/12

4/12-1/15=20/60-4/60

=16/60=4/15

2/3x15/8=30/24=5/4

2/3:8/21=42/24=7/4

nhớ cho a tích nhé =)) a bị bịp nhiều rồi

\(\frac{5}{11}+3\frac{8}{21}+\frac{6}{11}-\frac{8}{21}\)

\(=\frac{5}{11}+\frac{71}{21}+\frac{6}{11}-\frac{8}{21}\)

\(=\left(\frac{5}{11}+\frac{6}{11}\right)+\left(\frac{71}{21}-\frac{8}{21}\right)\)

\(=1+3\)

\(=4\)

Đúng lun, Mn tk mk nha!

\(\frac{5}{11}+3\frac{8}{21}+\frac{6}{11}-\frac{8}{21}\)

=\(\frac{5}{11}+\frac{71}{21}+\frac{6}{11}-\frac{8}{21}\)

=\(\left(\frac{5}{11}+\frac{6}{11}\right)+\left(\frac{71}{21}-\frac{8}{21}\right)\)

=\(1+3\)

=

a,  3 - 6 + 9 - 12 + 15 - 18 + 21

= 3 + 9 + 15 + 21 -6 -12- 18

=(21 - 18) + (15 - 12) + (9 - 6) +3

= 3 + 3 + 3 + 3

=3 *4 

= 12

b,, 48 :8 + 24 : 8 + 8 : 8

= (48 + 24 + 8) : 8 

= 80 : 8

= 10

k cho mình nha ....

a, 3 - 6 + 9 - 12 + 15 - 18 + 21 = 12

b, 48 : 8 + 24 : 8 + 8 : 8 = 10

a) \(\dfrac{3}{8}:\dfrac{9}{4}\)

\(=\dfrac{3}{8}\times\dfrac{4}{9}\)

\(=\dfrac{3\times4}{8\times9}\)

\(=\dfrac{1\times1}{2\times3}\)

\(=\dfrac{1}{6}\)

b) \(\dfrac{8}{21}:\dfrac{4}{7}\)

\(=\dfrac{8}{21}\times\dfrac{7}{4}\)

\(=\dfrac{8\times7}{21\times4}\)

\(=\dfrac{2\times1}{3\times1}\)

\(=\dfrac{2}{3}\)

c) \(\dfrac{5}{8}:\dfrac{15}{8}\)

\(=\dfrac{5}{8}\times\dfrac{8}{15}\)

\(=\dfrac{5\times8}{8\times15}\)

\(=\dfrac{1\times1}{1\times3}\)

\(=\dfrac{1}{3}\)

a: \(=\dfrac{3}{8}\cdot\dfrac{4}{9}=\dfrac{12}{72}=\dfrac{1}{6}\)

b: \(=\dfrac{8}{21}\cdot\dfrac{7}{4}=\dfrac{56}{84}=\dfrac{2}{3}\)

c: \(=\dfrac{5}{8}\cdot\dfrac{8}{15}=\dfrac{5}{15}=\dfrac{1}{3}\)

Phương pháp giải:

Nhẩm bảng nhân, chia trong phạm vi đã học rồi điền kết quả vào chỗ trống.

Lời giải chi tiết:

2x8=16
21:3=7
16:2=8
4x5=20 
học tốt

1: =5/8*17/13*7/17*8/3*6

=5/3*7/13*6

=5*7*2/13=70/13

2: =-7/9(24/19-1)

=-7/9*5/19

=-35/171

3: =-3/8(7/6-9/6)

=-3/8*(-2/6)

=3/8*1/3=1/8