a) 2/5 + 1/2 + 7/10

= 4/10 + 5/10 + 7/10

= 9/10 + 7/10

= 16/10 = 8/5

b) 4/9 + 11/8 - 5/6

= 32/72 + 99/72 - 60/72

= 191/72

c) 9/20 - 8/15 x 5/12

= 9/20 - 2/9

= 41/180

d) 2/3 : 4/5 : 7/12

= 5/6 : 7/12

= 10/7

a,2/5+1/2+7/10

=4/10+5/10+7/10

=16/10

=8/5

b,4/9+11/8-5/6

=32/72+99/72-5/6

=131/72-5/6

=131/72-60/72

=71/72

c,9/20-8/15x5/12

=9/20-2/9

=41/180

d,2/3:4/5:7/12

=2/5x5/4x12/7

=6/7

=\(\dfrac{27}{5}\)

\(\dfrac{12}{5}+3=\dfrac{12}{5}+\dfrac{15}{5}=\dfrac{12+15}{5}=\dfrac{27}{5}\)

12/5+3 = 12/5 + 15/5 = 27/5

\(\frac{4}{5}\)+ \(\frac{1}{5}\): \(\frac{-2}{15}\)= \(\frac{4}{5}\)+ \(\frac{1}{5}\)* \(\frac{-15}{2}\)=\(\frac{4}{5}\) + \(\frac{-3}{2}\)= \(\frac{-7}{10}\)

b = \(\frac{-7}{12}\). (\(\frac{3}{11}\)+ \(\frac{-14}{11}\)) + \(\frac{5}{6}\)= \(\frac{-7}{12}\). (-1) + \(\frac{5}{6}\)= \(\frac{7}{12}\)+ \(\frac{10}{12}\)= \(\frac{17}{12}\)

x-\(\frac{3}{5}\)x = \(\frac{2}{5}\)

=> x.(1-\(\frac{3}{5}\)) =\(\frac{2}{5}\)

=> x. \(\frac{2}{5}\)= \(\frac{2}{5}\)

=> x=1

Vậy x=1

a,\(\frac{2}{5}+\frac{3}{4}=\frac{8+15}{20}=\frac{23}{20}\)

\(\frac{7}{12}-\frac{2}{7}+\frac{1}{12}=(\frac{7}{12}+\frac{1}{12})-\frac{2}{7}\)

\(=\frac{2}{3}-\frac{2}{7}=\frac{14-6}{21}=\frac{8}{21}\)

\(\frac{12}{17}-\frac{5}{17}-\frac{4}{17}=\frac{7}{17}-\frac{4}{17}=\frac{3}{17}\)

b,\(675,39+342,14=1017,53\)

\(563,87+403,13-328,35=638,65\)

2/5+3/4=23/20

7/12-2/7+1/12=8/21

12/17-5/17-4/173/17

675,39+342,14=1017,53

563,87+403,13-328,35=638,65

( 48 x 5 + 48 x 7 - 48 x 12 )

=48 x ( 5 + 7 - 12 )

=48 x 0

= 0

\(\Rightarrow\)( 48 x 5 + 48 x 7 - 48 x 12 ) : ( 1 + 3 + 5 + 7 +.....+ 91 ) = 0

Hk tốt

      \(\left(48\text{ x }5+48\text{ x }7-48\text{ x }12\right)\text{ : }\left(1+3+5+7+...+91\right)\)

\(=\left(48\text{ x }5+48\text{ x }7-48\text{ x }12\right)\text{ : }A\)

\(=48\text{ x }\left(5+7-12\right)\text{ : }A\)

\(=48\text{ x }\left(12-12\right)\text{ : }A\)

\(=48\text{ x }0\text{ : }A\)

\(=0\text{ : }A\)

\(=0\)

(48 x 5+48 x 7 -48 x12):(1+3+5+7+...+91)

=(48x12-48x12):(91+1)x45:2

=0:2070

=0

Bài 1

\(\dfrac{1}{2}+\dfrac{7}{10}=\dfrac{5}{10}+\dfrac{7}{10}=\dfrac{12}{10}=\dfrac{6}{5}\)

\(\dfrac{8}{15}\times\dfrac{5}{12}=\dfrac{40}{180}=\dfrac{2}{9}\)

Baì 2 

\(30\times\dfrac{5}{3}=50\)

\(\dfrac{6}{5}\times\dfrac{5}{3}=2\)

\(\dfrac{12}{40}\times\dfrac{5}{3}=\dfrac{1}{2}\)

Bài 1:

1/2 + 7/10 = 10/20 + 14/20 = 6/5

8/15 x 5/12 = 2/9

Bài 2: 

5/3 của số 30 là: 30 x 5/3 = 50

5/3 của số 6/5 là: 6/5 x 5/3 = 2

5/3 của số 12/40 là: 12/40 x 5/3 = 1/2

\(\dfrac{1}{2}\)+\(\dfrac{7}{10}\)=\(\dfrac{5}{10}\)+\(\dfrac{7}{10}\)=\(\dfrac{12}{10}\)=\(\dfrac{6}{5}\)

Bài 1:

1/2  +  7/10 = 1, 2 

 8/15   x   5/12 = 2/9

1) 1/2+ 7/10 = 5/10+ 7/10=  12/10 = 6/5

8/15 x 5/12= 40/ 180 = 2/9

2) 5/3 của 30 là: 50

5/3 của 6/5 là: 2

5/3 của 12/40 là: 1/2

1/2  +  7/10 = 6/5

 8/15   x   5/12 = 2/9

a)2/5+3/10-1/2=7/10-1/2=2/10=1/5

b)8/11+8/33x3/4=8/11+2/11=10/11

c)7/9x3/14:5/8=1/6:5/8=8/30+4/15

d)5/12-7/32:21/16=5/12-1/6=3/12

nha

a) 2/5+3/10-1/2

= 4/10+3/10-5/10

= 2/10 = 1/5

b) 8/11+8/33x3/4

= 8/11+2/11

= 10/11

c) 7/9x3/14:5/8

= 1/6:5/8

= 1/6x8/5

=4/15

d) 5/12-7/32:21/16

= 5/12-7/32x16/21

= 5/12-1/6

= 5/12-2/12

= 3/12

nnl hhhhhhhh