1a. chọn mẫu chung là 120,ta có

\(\frac{7}{30}=\frac{7.4}{30.4}=\frac{28}{120}\)

\(\frac{13}{60}=\frac{13.2}{60.2}=\frac{26}{120}\)

\(\frac{-9}{40}=\frac{-9.3}{40.3}=\frac{27}{120}\)

vậy....

1b. chon mẫu số chung là 60

\(\frac{3}{-20}=\frac{-3.3}{20.3}=-\frac{9}{60}\)

\(\frac{-11}{-30}=\frac{-11.\left(-2\right)}{-30.\left(-2\right)}=\frac{22}{60}\)

\(\frac{7}{15}=\frac{7.4}{15.4}=\frac{28}{60}\)

2a.\(\frac{-15}{90}=\frac{-1}{6};\frac{120}{600}=\frac{1}{5};\frac{-75}{150}=\frac{-1}{2}\)

mẫu chung 30

\(\frac{-1}{6}=-\frac{5}{30};\frac{1}{5}=\frac{16}{30};-\frac{1}{2}=\frac{-15}{30}\)

vậy...

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hộ C1: Tìm x biết

a 2x-35=15

=>2x=15+35=50

=>x=25

b 3x+17=2

=>3x=2-17=-15

=>x=5

c x+3/15=1/3

x=1/3-3/15=2/15

d x-12/4=1/2

x=1/2+12/4=7/2

a. 2x-35=15

    2x=15+35

    2x=50

      x=50:2

      x=25

Vậy x=25

b. 3x+17=2

    3x=2-17

    3x=-15

      x=-15:3

      x=-5

Vậy x=-5

c. \(\frac{x+3}{15}=\frac{1}{3}\)

\(\Rightarrow3x+9=15\)

      \(3x=15-9\)

      \(3x=6\)

         \(x=6:3\)

         \(x=3\)

Vậy x=3

d. \(\frac{x-12}{4}=\frac{1}{2}\)

\(\Rightarrow2x-24=4\)

      \(2x=4+24\)

      \(2x=28\)

         \(x=28:2\)

         \(x=14\)

Vậy x=14

C1

a, 2x - 35 = 15 

2x = 15 + 35

2x = 50

x = 25

b, 3x + 17 = 2

3x = 2 - 17

3x = -15

x = -3

c, \(\frac{x+3}{15}=\frac{1}{3}\)

\(\frac{3x+3}{45}=\frac{15}{45}\)

3x + 3 = 15

3x = 15 - 3

3x = 12

x = 4

d, \(\frac{x-12}{4}=\frac{1}{2}\)

\(\frac{2x-24}{8}=\frac{4}{8}\)

2x - 24 = 4

2x = 4 + 24

2x = 28 

x = 14

B2

a , \(\frac{-315}{540}=\frac{-7}{12}\)  

b, \(\frac{25\times13}{26\times35}=\frac{5}{14}\)

c, \(\frac{6\times9-2\times17}{63\times3-119}=\frac{2}{7}\)

d, \(\frac{x-12}{4}=\frac{1}{2}\)

\(\frac{2x-24}{8}=\frac{4}{8}\)

2x - 24 = 4

2x = 28

x = 14 ( giống câu d bài 1 )

B3 

a, \(\frac{6}{7}=\frac{6\times5}{7\times5}=\frac{30}{35}\)

\(\frac{3}{5}=\frac{3\times7}{5\times7}=\frac{21}{35}\)

b, \(\frac{3}{-2}\)và \(\frac{-1}{-4}=\frac{1}{4}\)

\(\frac{3}{-2}=\frac{3\times-2}{-2\times-2}=\frac{-6}{4}\)

c, \(\frac{7}{-10}=\frac{7\times8}{-10\times8}=\frac{56}{-80}=\frac{-56}{80}\)

\(\frac{7}{8}=\frac{7\times-10}{8\times-10}=\frac{-70}{-80}=\frac{70}{80}\)

d, \(\frac{14}{21}=\frac{14\times72}{21\times72}=\frac{1008}{1512}\)

\(\frac{-60}{72}=\frac{-60\times21}{72\times21}=\frac{-1260}{1512}\)

1. Rút gọn phân số

a) \(\dfrac{-12}{21}\) = \(\dfrac{-12:3}{21:3}\) = \(\dfrac{-4}{7}\)

b) \(\dfrac{-63}{81}\) = \(\dfrac{-63:9}{81:9}\) = \(\dfrac{-7}{9}\)

2. Quy đồng mẫu số

\(\dfrac{-6}{5}\) \(\dfrac{8}{-4}\)

⇒ \(\dfrac{-6}{5}\) = \(\dfrac{-8}{4}\) MSC: 20

\(\dfrac{-6}{5}\) = \(\dfrac{-24}{20}\)

\(\dfrac{-8}{4}\) = \(\dfrac{-40}{20}\)

3. So sánh phân số

\(\dfrac{6}{7}\) và \(\dfrac{11}{10}\) MSC: 70

\(\dfrac{6}{7}\) = \(\dfrac{60}{70}\)

\(\dfrac{11}{10}\) = \(\dfrac{77}{70}\)

Vì \(\dfrac{60}{70}\) < \(\dfrac{77}{70}\) nên \(\dfrac{6}{7}\) < \(\dfrac{11}{10}\)

4. Cộng hai phân số

a) \(\dfrac{3}{4}\) + \(\dfrac{-7}{6}\) MSC: 12

\(\dfrac{3}{4}\) = \(\dfrac{9}{12}\)

\(\dfrac{-7}{6}\) = \(\dfrac{-14}{12}\)

\(\dfrac{9+\left(-4\right)}{12}\) = \(\dfrac{-5}{12}\)

Hình như sai

Câu 4

3/4, -7/6

27/81=1/3

57/76=3/4

35/84=5/12

27/81=1/3

57/76=3/4

35/84=5/12

567895464468457899999 hư đi em 

\(a.\dfrac{5}{10}=\dfrac{1}{2}=\dfrac{1\times3}{2\times3}=\dfrac{3}{6}\)

\(\dfrac{25}{75}=\dfrac{1}{3}=\dfrac{1\times2}{3\times2}=\dfrac{2}{6}\)

\(b.\dfrac{42}{56}=\dfrac{3}{4}=\dfrac{3\times2}{4\times2}=\dfrac{6}{8}\)

\(\dfrac{18}{48}=\dfrac{3}{8}\)

\(c.\dfrac{27}{81}=\dfrac{1}{3}=\dfrac{1\times4}{3\times4}=\dfrac{4}{12}\)

\(\dfrac{57}{76}=\dfrac{3}{4}=\dfrac{3\times3}{4\times3}=\dfrac{9}{12}\)

\(\dfrac{35}{84}=\dfrac{5}{12}\)

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a.3/6 va 2/6     b.24/32 va12/32  c.912/2736  2052/2736 1140/2736