Bài 5: Quy đồng mẫu nhiều phân số

a) \(\dfrac{1}{3};\dfrac{1}{2};\dfrac{2}{3};\dfrac{1}{4};\dfrac{3}{4};\dfrac{1}{6};\dfrac{5}{6}\)

quy đồng các mấu số trên có mẫu chung là 12

ta có : \(\dfrac{1}{3}=\dfrac{4}{12}\)

\(\dfrac{1}{2}=\dfrac{6}{12}\)

\(\dfrac{2}{3}=\dfrac{8}{12}\)

\(\dfrac{1}{4}=\dfrac{3}{12}\)

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

\(\dfrac{1}{6}=\dfrac{2}{12}\)

\(\dfrac{5}{6}=\dfrac{10}{12}\)

b) \(\dfrac{2}{-7};\dfrac{1}{9};\dfrac{-2}{-9};\dfrac{8}{-21};\dfrac{-10}{21}\)

quy đồng mấu số trên có mẫu chung là 63

ta có \(\dfrac{2}{-7}=\dfrac{18}{-63}\)

\(\dfrac{1}{9}=\dfrac{7}{63}\)

\(\dfrac{-2}{-9}=\dfrac{-14}{-63}\)

\(\dfrac{8}{-21}=\dfrac{24}{-63}\)

\(\dfrac{-10}{21}=\dfrac{-30}{63}\)

a: \(\dfrac{1}{3}=\dfrac{1\cdot4}{3\cdot4}=\dfrac{4}{12}\)

\(\dfrac{1}{2}=\dfrac{1\cdot6}{2\cdot6}=\dfrac{6}{12}\)

\(\dfrac{2}{3}=\dfrac{2\cdot4}{3\cdot4}=\dfrac{8}{12}\)

\(\dfrac{1}{4}=\dfrac{1\cdot3}{4\cdot3}=\dfrac{3}{12}\)

\(\dfrac{3}{4}=\dfrac{3\cdot3}{4\cdot3}=\dfrac{9}{12}\)

\(\dfrac{1}{6}=\dfrac{1\cdot2}{6\cdot2}=\dfrac{2}{12}\)

\(\dfrac{5}{6}=\dfrac{5\cdot2}{6\cdot2}=\dfrac{10}{12}\)

b: \(\dfrac{2}{-7}=\dfrac{-2}{7}=\dfrac{-2\cdot9}{7\cdot9}=\dfrac{-18}{63}\)

\(\dfrac{1}{9}=\dfrac{1\cdot7}{9\cdot7}=\dfrac{7}{63}\)

\(\dfrac{-2}{-9}=\dfrac{2}{9}=\dfrac{2\cdot7}{9\cdot7}=\dfrac{14}{63}\)

\(\dfrac{8}{-21}=\dfrac{-8}{21}=\dfrac{-8\cdot3}{21\cdot3}=\dfrac{-24}{63}\)

\(\dfrac{-10}{21}=\dfrac{-10\cdot3}{21\cdot3}=\dfrac{-30}{63}\)

3/14=264/1232

2/11=224/1232

7/16=539/1232

=>7/16 lớn nhất

=>Ngày 3 sửa được nhiều nhất

xin mọi người             

5/24=15/72

7/12=42/72

7/18=28/72

SOS

5/24=5/24=15/72

7/12=14/24=42/72

7/18=28/72

`5/24 = 15/72`.

`7/12 = 42/72`.

`7/18 = 28/72`.

\(\Leftrightarrow\dfrac{x+2}{7}=\dfrac{y-1}{14}=\dfrac{-3}{7}\)

=>x+2=-3; y-1=-6

=>x=-5; y=-5

`+), 35y - 35 = -15 xx 14`

` 35y - 35 = -210`

` 35y = -175`

` y = -5`

`+), 35x + 70 = - 105`

`=> 35x = -175`

`=> x = -5`.

Xét \(\dfrac{4}{x}=\dfrac{28}{49}\Rightarrow\dfrac{28}{7x}=\dfrac{28}{49}\Leftrightarrow7x=49\Leftrightarrow x=7\)

\(\dfrac{y}{21}=\dfrac{28}{49}\Rightarrow49y=28.21\Leftrightarrow y=\dfrac{28.21}{49}=\dfrac{7.4.7.3}{7.7}=12\)

a: -51/136=-3/8

-60/108=-5/9

26/-156=-26/156=-1/6

b: -165/270=-11/18

-91/-156=7/12

-210/1134=-5/27