Bài 5:
1.
a, 10 + $\frac{5}{4}$ . 5 = 10 + $\frac{25}{4}$ = $\frac{65}{4}$
b, 2² . $\frac{9}{27}$ . 2 = 4 . $\frac{1}{3}$ . 2 = $\frac{8}{3}$
2.
A = $\frac{1}{1.3}$ + $\frac{1}{3.5}$ + $\frac{1}{5.7}$ + ... + $\frac{1}{2019.2021}$
A = 1 - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{7}$ + ... + $\frac{1}{2019}$ - $\frac{1}{2021}$
A = 1 - $\frac{1}{2021}$
A = $\frac{2020}{2021}$