a,$\frac{5}{7}$ . $\frac{4}{9}$ . $\frac{7}{5}$ . $\frac{9}{-6}$
=($\frac{5}{7}$ . $\frac{7}{5}$) . ( $\frac{4}{9}$.$\frac{9}{-6}$)
= 1 . $\frac{4}{-6}$
= $\frac{-4}{6}$
b,$\frac{33}{27}$ . $\frac{81}{99}$ . $\frac{104}{62}$ .$\frac{124}{32}$
= $\frac{1}{1}$ . $\frac{3}{3}$ . $\frac{52}{31}$ . $\frac{31}{8}$
= 1 . $\frac{52}{8}$
= $\frac{52}{8}$
CHÚC BẠN HỌC TỐT!!!!