7 x 2 = 14       7 x 4 = 28       7 x 6 = 42       7 x 3 = 21

2 x 7 = 14       4 x 7 = 28       6 x 7 = 42       3 x 7 = 21

14 : 7 = 2       28 : 7 = 4       42 : 7 = 6       21 : 7 = 3

14 : 2 = 7       28 : 4 = 7       42 : 6 = 7       21 : 3 = 7

\(\dfrac{21}{17}\times\dfrac{13}{14}\times56\times\dfrac{3}{42}\)
\(=\dfrac{21}{17}\times\dfrac{13}{14}\times56\times\dfrac{1}{14}\)
\(=\dfrac{21\times13\times56}{17\times14\times14}\)
\(=\dfrac{3\times7\times13\times14\times2\times2}{17\times2\times7\times14}\)
\(=\dfrac{3\times13\times2}{17}\)
\(=\dfrac{78}{17}\)

1) 42/35 - [ 33/14 . 21/22 - ( 54/25 : 27/25 - 7/10 )]

= 42/35 - ( 9/4-13/10)

=42/35-19/20

=1/4

2) a) 5/21 . x/34 = 35/102     

x/34=35/102 : 5/21

x/34=49/34

=> x=34

Vậy x=34

b) 15/x . 21/13 = 45/91

15/x=45/91 : 21/13

15/x=15/49

=> x=49

Vậy x=49

lớp 4

\(\dfrac{\left(-9\right)\times14}{21\times\left(-45\right)}\)

\(=\dfrac{-126}{-945}=\dfrac{2}{15}\)

-----------------------

\(\dfrac{\left(-12\right)\times14}{\left(-42\right)\times\left(-6\right)}\)

\(=\dfrac{-168}{252}=\dfrac{-2}{3}\)

-6480

\(\left(-9\right)\times\dfrac{14}{21}\times\left(-45\right)=\left(-9\right)\times\dfrac{2}{3}\times\left(-45\right)=\dfrac{-9}{1}\times\dfrac{2}{3}\times\left(-45\right)=\dfrac{-18}{3}\times\dfrac{-45}{1}=\dfrac{810}{3}=270\)

\(\left(-12\right)\times\dfrac{14}{-42}\times\left(-6\right)=\dfrac{-12}{1}\times\dfrac{-1}{3}\times\left(-6\right)=\dfrac{12}{3}\times\left(-6\right)=4\times\left(-6\right)=-24\)

\(\frac{6}{18}+\frac{-14}{21}=\frac{1}{3}+\frac{-2}{3}=\frac{-1}{3}\);     ; 

\(\frac{6}{13}+\frac{-14}{39}=\frac{18}{39}+\frac{-14}{39}=\frac{4}{39}\);

\(\frac{-3}{21}+\frac{6}{42}=\frac{-1}{7}+\frac{1}{7}=0\); 

\(\frac{7}{21}+\frac{9}{-36}=\frac{1}{3}+\frac{-1}{4}=\frac{4}{12}+\frac{-3}{12}=\frac{1}{12}\)

a) 7 x 5 = 35       7 x 6 = 42       7 x 8 = 56       7 x 4 = 28

    35 : 7 = 5       42 : 7 = 6       56 : 7 = 8       28 : 7 = 4

b) 42 : 7 = 6       35 : 7 = 5       63 : 7 = 9       16 : 2 = 8

    14 : 7 = 2       49 : 7 = 7       7 : 7 = 1       48 : 6 = 8

    28 : 7 = 4       70 : =10       21 : 7 = 3       56 : 7 = 8

 7 X 5 = 35

7 x 5 = 35 7 x 6 = 42 7 x 8 = 56 7 x 4 = 28 35 : 7 = 5 42 : 7 = 6 56 : 7 = 8 28 : 7 = 4 b) 42 : 7 = 6 35 : 7 = 5 63 : 7 = 9 16 : 2 = 8 14 : 7 = 2 49 : 7 = 7 7 : 7 = 1 48 : 6 = 8 28 : 7 = 4 70 : =10 21 : 7 = 3 56 : 7 = 8

b,21/17x13/14x56x3/42

=21/17x3/42x56x13/14

=21x3/17x42 x 56x13/14

=21x3/17x21x2 x 14x4x13/14

=3/17x2 x 4x13

=3/24 x 52=3x52/24

=3x52/3x8

=52/8=13/2

a, 6/7 x 16/15 x 7/6 x 21/32

=(6/7 x 7/6)x16/15x21/32

=16/15x21/32

=16x21/15x32

=16x3x7/3x5x16x2

=7/5x2=7/10

a)(2800+14):14

=2814:14

=201

b)(2100-42):21

=2058:21

=98

a) (2800+14):14

=2814:14

=201

b) (2100-42):21

=2058:21

=98

Ủng hộ mk nka!!^_^

a)(2800+14):14=2814:14=21

b)(2100-42):21=2058:21=98