a) $\frac{-13}{14}$ = $\frac{-13.15}{14.15}$ = $\frac{-195}{210}$
.
$\frac{-14}{15}$ = $\frac{-14.14}{15.14}$ = $\frac{-196}{210}$
.
Mà $\frac{-195}{210}$ > $\frac{-196}{210}$
=> $\frac{-13}{14}$ > $\frac{-14}{15}$
.
b) $\frac{11}{18}$ = $\frac{11.7}{18.7}$ = $\frac{77}{126}$
.
$\frac{17}{21}$ = $\frac{17.6}{21.6}$ = $\frac{102}{126}$
.
Mà $\frac{102}{126}$ > $\frac{77}{126}$
=> $\frac{17}{21}$ > $\frac{11}{18}$