-25/20 < 20/25

15/21 > 21/49

Lời giải:
a. $\frac{3}{-7}=\frac{-27}{63}$

$\frac{-5}{9}=\frac{-35}{63}$

Do $\frac{27}{63}< \frac{35}{63}$ nên $\frac{-27}{63}> \frac{-35}{63}$

$\Rightarrow \frac{3}{-7}> \frac{-5}{9}$

---------

b.

$-0,625=\frac{-625}{1000}=\frac{-5}{8}=\frac{-125}{200}$

$\frac{-19}{50}=\frac{-76}{200}> \frac{-125}{200}$

$\Rightarrow -0,625> \frac{-19}{50}$

c.

$-2\frac{5}{9}=-(2+\frac{5}{9})=\frac{-23}{9}=-(\frac{-23}{-9})$

1.ta có:30/47=30.59/47.59=1770/2773;41/59=41.47/59/47=1927/2773

Vì 1770/2773 nhỏ hơn 1927/2773

suy ra 30/47 nhỏ hơn 41/59

2.ta có:24/49=24.45/49.45=1080/2205;23/45=23.49/45.49=1035/2205

vì 1080/2205 lớn hơn 1035/2205

suy ra 24/49 lớn hơn 23/45

1:< 2:>

1)

-2/15 < 0 

-10/-11 > 0

nên x < y

2) 

ví dụ 1 < 2 => -1 > -2

ta có 16/29 < 16/27 < 19/27 

suy ra -16/29 > -16/27 > -19/27

x = 2/7 = 22/77

y =3/11 = 21/77

Vì 22>21 nên 22/77 > 21/77

Vậy 2/7 > 3/11

\(a,\dfrac{-15}{17}=-1+\dfrac{2}{17}\\ -\dfrac{19}{21}=-1+\dfrac{2}{21}\\ Vì:\dfrac{2}{17}>\dfrac{2}{21}\Rightarrow-1+\dfrac{2}{17}>-1+\dfrac{2}{21}\Rightarrow-\dfrac{15}{17}>-\dfrac{19}{21}\\ b,-\dfrac{24}{35}=-1+\dfrac{11}{35};-\dfrac{19}{30}=-1+\dfrac{11}{30}\\ Vì:\dfrac{11}{35}< \dfrac{11}{30}\Rightarrow-1+\dfrac{11}{35}< -1+\dfrac{11}{30}\\ \Rightarrow-\dfrac{24}{35}< -\dfrac{19}{30}\)

a: \(\dfrac{12}{49}< \dfrac{13}{49}< \dfrac{13}{47}\)

b: \(\dfrac{12}{47}>\dfrac{19}{47}>\dfrac{19}{77}\)

a/ so sánh bằng 12/47

b/ chưa tìm ra

ta có:      -157/623=1-(780/623)

              -47/213=1-(260/213)

ta thấy 260/213=780/639<780/623

=>1-260/213>1-780/623

=>-47/213>-157.623

ta có:      -157/623=1-(780/623)

              -47/213=1-(260/213)

ta thấy 260/213=780/639<780/623

=>1-260/213>1-780/623

=>-47/213>-157.623