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

a) Sai;     b) Sai;     c) Đúng;     d) Sai

bài 1 . 10

bài 2 . 12

bài 3 . 6

bài 4 .  20/14

bài 5 . 5/20

mình chỉ làm được vậy thôi

a)  3 8 + 5 − 8 = 3 8 + − 5 8 = 3 + − 5 8 = − 2 8 = − 1 4

b)  − 3 7 + 5 − 7 = − 3 7 + − 5 7 = − 3 + − 5 7 = − 8 7

a: \(\dfrac{2}{3}+\dfrac{7}{3}=\dfrac{9}{3}\)

\(\dfrac{7}{3}+\dfrac{2}{3}=\dfrac{9}{3}\)

=>\(\dfrac{2}{3}+\dfrac{7}{3}=\dfrac{7}{3}+\dfrac{2}{3}\)

\(\dfrac{3}{5}+\dfrac{4}{5}=\dfrac{7}{5}\)

\(\dfrac{4}{5}+\dfrac{3}{5}=\dfrac{7}{5}\)

=>\(\dfrac{3}{5}+\dfrac{4}{5}=\dfrac{4}{5}+\dfrac{3}{5}\)

b: \(\dfrac{7}{9}+\dfrac{16}{9}=\dfrac{7+16}{9}=\dfrac{23}{9}\)

\(\dfrac{16}{9}+\dfrac{7}{9}=\dfrac{16+7}{9}=\dfrac{23}{9}\)

Do đó: \(\dfrac{7}{9}+\dfrac{16}{9}=\dfrac{16}{9}+\dfrac{7}{9}\)

la phan so 3/5 nha ban

Phân số đó là :4/5

\(\left(\frac{1}{3}+\frac{2}{7}+\frac{5}{4}\right):3=\frac{7\times4+2\times3\times4+5\times3\times7}{3\times7\times4}\times\frac{1}{3}\)

                                         \(=\frac{28+24+105}{84}\times\frac{1}{3}\)

                                          \(=\frac{157}{84}\times\frac{1}{3}\)
                                          \(=\frac{157}{252}\)

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