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a) Ta có: a = -1/8 = -9/72
b = 2/-9 = -2/9 = -16/72
Ta thấy: -9 > -16 => -9/72 > -16/72
hay a > b
Vậy a > b
b) Ta có: a = 12/15 = 4/5= 16/20
b = -( -3/4 ) = 3/4= 15/20
Ta thấy: 16 > 15 => 16/20 > 15/20
hay a > b
Vậy a > b
c) Ta có: a = -2/3 = -40/60
b = -0,65 = -13/20 = -39/60
Ta thấy: -40 < -39 => -40/60 < -39/60
hay a < b
Vậy a < b
d) Ta có: a = -21/3 = -7
b = -413% = -4,13
Ta thấy: -7 < -4,13
=> a < b
Vậy a < b
Chuk bn hok tốt! 
a, 4\9 < 13\18
b,-15\7 < -6\5
c,278\37 > 287\46
d,-157\623 < -47\213
\(\frac{11}{13}\)và \(\frac{22}{27}\)
Ta có:
\(\frac{11}{13}=\frac{297}{351}\)
\(\frac{22}{27}=\frac{242}{351}\)
Mà: \(\frac{297}{351}>\frac{242}{351}\)
Vậy \(\frac{11}{13}>\frac{22}{27}\)
\(\frac{-5}{11}\)và \(\frac{-9}{23}\)
Ta có:
\(\frac{-5}{11}=\frac{-115}{253}\)
\(\frac{-9}{23}=\frac{-99}{253}\)
Mà: \(\frac{-115}{253}< \frac{-99}{253}\)
Vậy \(\frac{-5}{11}< \frac{-9}{23}\)
a; \(\dfrac{4}{27}\) = \(\dfrac{4\times7}{27\times7}\) = \(\dfrac{28}{189}\)
\(\dfrac{15}{63}\) = \(\dfrac{15\times3}{63\times3}\) = \(\dfrac{45}{189}\)
\(\dfrac{28}{189}\) < \(\dfrac{45}{189}\)
- \(\dfrac{28}{189}\) > - \(\dfrac{45}{189}\)
Vậy - \(\dfrac{4}{27}\) > \(\dfrac{15}{-63}\)
b; \(\dfrac{13}{15}\) = \(\dfrac{13\times9}{15\times9}\) = \(\dfrac{117}{135}\)
\(\dfrac{9}{11}\) = \(\dfrac{9\times13}{11\times13}\) = \(\dfrac{117}{143}\)
\(\dfrac{117}{135}\) > \(\dfrac{117}{143}\)
Vậy \(\dfrac{13}{15}\) > \(\dfrac{9}{11}\)