a) \(-\dfrac{8}{31}=\dfrac{-8\cdot101}{31\cdot101}=\dfrac{-808}{3131}\)

\(\dfrac{-789}{3131}=\dfrac{-789}{3131}\)

c) \(\dfrac{1}{n}=\dfrac{n+1}{n\left(n+1\right)}\)

\(\dfrac{1}{n+1}=\dfrac{n}{n\left(n+1\right)}\)

a) \(-\dfrac{8}{31}=\dfrac{-8\cdot101}{31\cdot101}=\dfrac{-808}{3131}\)

\(\dfrac{-789}{3131}=\dfrac{-789}{3131}\)

b) \(\dfrac{11}{2^3\cdot3^4\cdot4^5}=\dfrac{11\cdot2^3\cdot5^3}{2^6\cdot3^4\cdot4^5\cdot5^3}=\dfrac{11000}{2^6\cdot3^4\cdot4^5\cdot5^3}\)

\(\dfrac{29}{2^2\cdot2^4\cdot5^3}=\dfrac{29\cdot3^4\cdot4^5}{2^6\cdot3^4\cdot4^5\cdot5^3}=\dfrac{2405376}{2^6\cdot3^4\cdot4^5\cdot5^3}\)

c) \(\dfrac{1}{n}=\dfrac{n+1}{n\left(n+1\right)}\)

\(\dfrac{1}{n+1}=\dfrac{n}{n\left(n+1\right)}\)

a: 7/15=56/120

-3/8=-45/120

-2/3=-80/120

a: 5/14=15/42

4/21=8/42

b: 4/5=48/60

7/12=35/60

8/15=32/60

a: 

13/17=1-4/17

8/12=1-4/12

mà 4/17<4/12

nên 13/17>8/12=12/18

b: 16/51<17/51=1/3=30/90<31/90

12/18<13/17

16/51>31/90

`MSC: 756`

Có: `5/21=[5.36]/[21.36]=180/756`

      `3/28=[3.27]/[28.27]=81/756`

      `45/[-108]=[-45.7]/[108.7]=[-315]/756`

Quy đồng \(\dfrac{5}{21};\dfrac{3}{28};\dfrac{45}{-108}\) 

Chuyển dấu \(\dfrac{45}{-108}=\dfrac{45 . \left(-1\right)}{\left(-108\right) . \left(-1\right)}=-\dfrac{45}{108}\) 

Ta có MSC(21,28,108): 756

\(\dfrac{5}{21}=\dfrac{5 . 36}{21 . 36}=\dfrac{180}{756}\) ; \(\dfrac{3}{28}=\dfrac{3 . 27}{28 . 27}=\dfrac{81}{756}\) ; \(-\dfrac{45}{108}=\dfrac{\left(-45\right) . 7}{108 . 7}=-\dfrac{315}{756}\)

a: -5/5=-1=-7/7

8/7=8/7

b: -3/15=-1/5=-6/30

5/6=25/30

c: -34/136=-1/4=-9/36

-12/108=-1/9=-4/36

26/-156=-1/6=-6/36

Và so sánh nữa nha ! mình cảm ơn các bạn !