a)12/21 và 3/21
b)12/36 và 15/36
c)63/56 và 48/56
Chúc bạn học tốt

5/7>6/21

18/24>15/25

12/13=1212/1313

\(\dfrac{5}{7}\)>\(\dfrac{6}{21}\)

\(\dfrac{18}{24}\)>\(\dfrac{15}{25}\)

\(\dfrac{12}{13}\)=\(\dfrac{1212}{1313}\)

a) 

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

\(\Rightarrow\dfrac{1}{2}=\dfrac{1\times2}{2\times2}=\dfrac{2}{4}\)

Mà \(\dfrac{2}{4}< \dfrac{3}{4}\)

Vậy \(\dfrac{6}{12}< \dfrac{3}{4}\).

b)

\(\dfrac{8}{10}=\dfrac{8:2}{10:2}=\dfrac{4}{5}\)

Mà \(\dfrac{2}{5}< \dfrac{4}{5}\)

Vậy \(\dfrac{2}{5}< \dfrac{8}{10}\).

c)

\(\dfrac{40}{35}=\dfrac{40:5}{35:5}=\dfrac{8}{7}\)

Mà \(\dfrac{8}{7}>\dfrac{6}{7}\)

Vậy \(\dfrac{40}{35}>\dfrac{6}{7}\).

d)

\(\dfrac{8}{16}=\dfrac{8:8}{16:8}=\dfrac{1}{2}\)

Mà \(\dfrac{1}{2}< \dfrac{5}{2}\)

Vậy \(\dfrac{8}{16}< \dfrac{5}{2}\).

Câu 3:

a) \(\dfrac{12}{36}=\dfrac{12:12}{36:12}=\dfrac{1}{3}\)

\(\dfrac{-16}{20}=\dfrac{-16:4}{20:4}=\dfrac{-4}{5}\)

b) \(\dfrac{21}{105}=\dfrac{21:21}{105:21}=\dfrac{1}{5}\)

\(\dfrac{35}{150}=\dfrac{35:5}{150:5}=\dfrac{7}{30}\)

Câu 4: 

a) \(\dfrac{3}{10}+\dfrac{5}{10}=\dfrac{3+5}{10}=\dfrac{8}{10}=\dfrac{4}{5}\)

b) Ta có: \(\left(-27\right)\cdot36+64\cdot\left(-27\right)+23\cdot\left(-100\right)\)

\(=\left(-27\right)\cdot\left(64+36\right)+23\cdot\left(-100\right)\)

\(=-27\cdot100-23\cdot100\)

\(=100\left(-27-23\right)\)

\(=-50\cdot100=-5000\)

c) \(\dfrac{5}{8}+\dfrac{3}{12}=\dfrac{15}{24}+\dfrac{6}{24}=\dfrac{21}{24}=\dfrac{7}{8}\)

d) Ta có: \(\dfrac{-2}{17}+\dfrac{3}{19}+\dfrac{-15}{17}+\dfrac{16}{19}+\dfrac{5}{6}\)

\(=\left(-\dfrac{2}{17}+\dfrac{-15}{17}\right)+\left(\dfrac{3}{19}+\dfrac{16}{19}\right)+\dfrac{5}{6}\)

\(=-1+1+\dfrac{5}{6}\)

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

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Bài 2: So sánh các phân số sau

7/15 > 5/13              11/12 > 7/48        24/35 > 21/7           15/26 > 7/8

á cái anh này muốn lên giường không

Đáp án : C . 5/24 và 7/25     

Hồ Khánh Ly đúng đấy các bạn

a; \(\dfrac{9}{27}\) + \(\dfrac{7}{-49}\)

= \(\dfrac{1}{3}\) - \(\dfrac{1}{7}\)

= \(\dfrac{7}{21}\) - \(\dfrac{3}{21}\)

= \(\dfrac{4}{21}\)

b; - \(\dfrac{12}{10}\) + \(\dfrac{-25}{30}\)

   =  - \(\dfrac{6}{5}\) - \(\dfrac{5}{6}\)

   = -\(\dfrac{36}{30}\) - \(\dfrac{25}{30}\)

  = \(\dfrac{-61}{30}\) 

c; \(\dfrac{-20}{35}\) + \(\dfrac{-16}{-24}\)

 =  - \(\dfrac{4}{7}\) + \(\dfrac{2}{3}\)

= - \(\dfrac{12}{21}\) + \(\dfrac{14}{21}\)

=  \(\dfrac{2}{21}\)

d; - \(\dfrac{21}{77}\) + \(\dfrac{10}{-35}\)

 = - \(\dfrac{3}{11}\) - \(\dfrac{2}{7}\)

 = - \(\dfrac{21}{77}\) -  \(\dfrac{22}{77}\)

= - \(\dfrac{43}{77}\)

a ) − 2 5 b ) 1 6 c ) − 3 4

d ) − 3 4 e ) − 1 f ) 1 2