a) Đặt A= $\frac{1}{3}$ + $\frac{1}{15}$ + $\frac{1}{35}$ + $\frac{1}{63}$ + $\frac{1}{99}$ + $\frac{1}{143}$ + $\frac{1}{195}$
2 A =$\frac{2}{1 x 3}$ + $\frac{2}{3 x 5}$ + $\frac{2}{7 x 5}$ + $\frac{2}{9 x7}$ + $\frac{2}{9 x 9}$ + $\frac{2}{9 x 13}$ + $\frac{2}{13 x 15}$
2A = $\frac{1}{1}$ -$\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{5}$- $\frac{1}{7}$ + $\frac{1}{7}$ - $\frac{1}{9}$ + $\frac{1}{9}$ - $\frac{1}{13}$ + $\frac{1}{13}$ - $\frac{1}{15}$
2A = 1 - $\frac{1}{15}$ = $\frac{14}{15}$
A = $\frac{14}{15}$ : 2 = $\frac{7}{15}$
b) $\frac{1414+1515+1616+1717+1818+1919}{2020+2121+2222+2323+2424+2525}$
= $\frac{11 x (14+15+16+17+18+19)}{11 x (20+21+22+23+24+25)}$
=$\frac{14+15+16+17+18+19}{20+21+22+23+24+25}$
=$\frac{99}{135}$
=$\frac{11}{15}$
