Ta có: \(\dfrac{5}{7} = \dfrac{{5.4}}{{7.4}} = \dfrac{{20}}{{28}}\) và \(\dfrac{{ - 3}}{4} = \dfrac{{ - 3.7}}{{4.7}} = \dfrac{{ - 21}}{{28}}\)

Như vậy, \(\dfrac{{20}}{{28}} + \dfrac{{ - 21}}{{28}} = \dfrac{{20 + \left( { - 21} \right)}}{{28}} =  \dfrac{-1}{{28}}\)

\(\hept{\begin{cases}\frac{4}{7}=\frac{4.12}{7.12}=\frac{48}{84}\\\frac{5}{12}=\frac{5.7}{12.7}=\frac{35}{84}\end{cases}}\)

bài3

1. \(\frac{4}{3}\)= \(\frac{4x4}{3x4}\)=\(\frac{20}{12}\)

\(\frac{3}{4}\)=\(\frac{3x3}{4x3}\)=\(\frac{9}{12}\)

2.\(\frac{5}{4}\)=\(\frac{5x6}{4x6}\)=\(\frac{30}{24}\)

\(\frac{5}{6}\)=\(\frac{5x4}{6x4}\)=\(\frac{20}{24}\)

3.\(\frac{3}{8}\)=\(\frac{3x6}{8x6}\)=\(\frac{18}{48}\)

\(\frac{1}{6}\)=\(\frac{1x6}{6x8}\)=\(\frac{6}{48}\)

4.\(\frac{2}{5}\)=\(\frac{2x7}{5x7}\)=\(\frac{14}{35}\)

\(\frac{4}{7}\)=\(\frac{4x5}{7x5}\)=\(\frac{20}{35}\)

TL

Phân số là 3/45/4

HT

Sao lại thế

Bài 3 : 

a: \(MSC:12\\ \dfrac{4}{3}=\dfrac{4\times4}{3\times4}=\dfrac{16}{12};\dfrac{3}{4}=\dfrac{3\times3}{3\times4}=\dfrac{9}{12}\)

b: \(MSC:12\\ \dfrac{5}{4}=\dfrac{5\times3}{4\times3}=\dfrac{15}{12};\dfrac{5}{6}=\dfrac{5\times2}{6\times2}=\dfrac{10}{12}\)

c:\(MSC:24\\ \dfrac{3}{8}=\dfrac{3\times3}{8\times3}=\dfrac{9}{24};\dfrac{1}{6}=\dfrac{1\times2}{6\times2}=\dfrac{2}{12}\)

d: \(MSC:35\\ \dfrac{2}{5}=\dfrac{2\times7}{5\times7}=\dfrac{14}{35};\dfrac{4}{7}=\dfrac{4\times5}{7\times5}=\dfrac{20}{35}\)

Bài 3: 

a: 4/3=16/12

3/4=9/12

b: 5/4=30/24

5/6=20/24

c: 3/8=18/48

1/6=8/48

d: 2/5=14/35

4/7=20/35

Bài 4: 

a: 7/12=28/48

5/48=5/48

b: 4/100=4/100

8/25=32/100

20/35

48/84 va 35/84 mình chắc chắn

\(\frac{4}{7}\)và \(\frac{5}{12}\):

Ta có:

\(\frac{4}{7}=\frac{4\times12}{7\times12}=\frac{48}{84}\);\(\frac{5}{12}=\frac{5\times7}{12\times7}=\frac{35}{84}\).

Vậy: Quy đồng mẫu số của \(\frac{4}{7}\)và \(\frac{5}{12}\)được: \(\frac{48}{84}\)và \(\frac{35}{84}\).

Nhớ:

\(\frac{a}{b}=\frac{a\times d}{b\times d}=\frac{ad}{bd}\),\(\frac{c}{d}=\frac{c\times b}{d\times b}=\frac{cb}{db}\)và \(\frac{a}{b}=\frac{a\times\left(d\div b\right)}{b\times\left(d\div b\right)}=\frac{ae}{be}\)và \(d⋮b=e\).

48  /84 và 35/84

MSC:60 NHA BẠN 

ko bé ơi 

lớp tui có bạn cũng tên Nguyễn Trung Hiếu

\(\frac{1}{3}\)=\(\frac{1x7}{3x7}\)=\(\frac{7}{21}\)

\(\frac{5}{7}\)=\(\frac{5x3}{5x7}\)=\(\frac{15}{21}\)

1/3= 1x7 / 3 x 7 = 7/21

5/7= 5x 3/ 7x 3 = 15/21 

phép tính này có chung mẫu số là 21

7/21,   15/21

a)\(\frac{5}{12}=\frac{5\times5}{12\times5}=\frac{25}{60}\)

\(\frac{7}{15}=\frac{7\times4}{15\times4}=\frac{28}{60}\)

b)\(\frac{2}{7}=\frac{2\times9}{7\times9}=\frac{18}{63}\)

\(\frac{4}{9}=\frac{4\times7}{9\times7}=\frac{28}{63}\)