a) $\frac{3}{5}$ - $\frac{7}{45}$ .... - $\frac{23}{21.25}$
= $\frac{1+5}{5}$ : 2 - $\frac{9+5}{45}$ : 2 + ..... - $\frac{21+25}{21.25}$ : 2
= ($\frac{1}{5}$ + $\frac{5}{5}$ - $\frac{9}{45}$ - $\frac{5}{45}$ + .... - $\frac{21}{21.25}$ - $\frac{25}{21.25}$ ) : 2
= ($\frac{1}{5}$ + 1 - $\frac{1}{5}$ - $\frac{1}{9}$ + $\frac{1}{13}$ + $\frac{1}{9}$ - $\frac{1}{17}$ - $\frac{1}{13}$ + $\frac{1}{17}$ + $\frac{1}{21}$ - $\frac{1}{21}$ + $\frac{1}{25}$ ) : 2
= ( 1 + $\frac{1}{25}$ ) : 2 = $\frac{26}{25}$ : 2 = $\frac{13}{25}$
b)
Anh phân tích dưới mẫu trước:
2020.2021 - 2011.2020 = 2020.(2021 - 2011) = 2020.10
Ở tử có:
2019.2020 + 2021.10 + 2010 = 2019.2020 + (2020 + 1).10 + 2010
= 2019.2020 + 2020.10 + 10 + 2010
= 2029.2020 + 2020
= 2030.2020
= 203.2020.10
Ta có: $\frac{203.2020.10}{2020.10}$ = 203