a: \(\frac{-7}{5}<\frac{-5}{5}=-1\) (1)
Ta có: 4<7<9
=>\(\frac14>\frac17>\frac19\)
=>\(\frac14-1>\frac17-1>\frac19-1\)
=>\(-\frac34>-\frac67>-\frac89\)
=>\(-1<-\frac89<-\frac67<-\frac34\) (2)
Ta có: \(\frac{-3}{4}=\frac{-3\cdot15}{4\cdot15}=\frac{-45}{60}\)
\(\frac45=\frac{4\cdot12}{5\cdot12}=\frac{48}{60}\)
\(\frac56=\frac{5\cdot10}{6\cdot10}=\frac{50}{60}\)
mà -45<48<50
nên \(-\frac34<\frac45<\frac56\) (3)
Từ (1),(2),(3) suy ra \(-\frac75<-\frac89<-\frac67<-\frac34<\frac45<\frac56\)
b: \(\frac{21}{20}=\frac{21\cdot7}{20\cdot7}=\frac{147}{140};\frac{5}{-4}=\frac{-5\cdot35}{4\cdot35}=\frac{-175}{140}\)
\(\frac{-15}{14}=\frac{-15\cdot10}{14\cdot10}=\frac{-150}{140};\frac{-43}{35}=\frac{-43\cdot4}{35\cdot4}=\frac{-172}{140}\)
\(\frac{-8}{-5}=\frac85=\frac{8\cdot28}{5\cdot28}=\frac{224}{140}\) ; \(\frac{1}{-7}=\frac{1\cdot\left(-20\right)}{\left(-7\right)\cdot\left(-20\right)}=\frac{-20}{140}\)
mà -175<-172<-150<-20<147<224
nên \(\frac{5}{-4}<-\frac{43}{35}<-\frac{15}{14}<\frac{1}{-7}<\frac{21}{20}<\frac{-8}{-5}\)
