Đặt $\frac{1}{3}$ + $\frac{1}{8}$ + $\frac{1}{15}$+ $\frac{1}{24}$+ $\frac{1}{35}$ + $\frac{1}{48}$ + $\frac{1}{63}$ + $\frac{1}{80}$ = B
=> 2B = $\frac{2}{3}$ + $\frac{2}{8}$ + $\frac{2}{15}$+ $\frac{2}{24}$+ $\frac{2}{35}$ + $\frac{2}{48}$ + $\frac{2}{63}$ + $\frac{2}{80}$
= $\frac{2}{1x3}$ + $\frac{2}{2x4}$ + $\frac{2}{3x5}$+ $\frac{2}{4x6}$+ $\frac{2}{5x7}$ + $\frac{2}{6x8}$ + $\frac{2}{7x9}$ + $\frac{2}{8x10}$
= 1 - $\frac{1}{3}$ + $\frac{1}{2}$ - $\frac{1}{4}$ + $\frac{1}{3}$ - $\frac{1}{5}$ + $\frac{1}{4}$ - $\frac{1}{6}$ + $\frac{1}{5}$ - $\frac{1}{7}$+ $\frac{1}{6}$ - $\frac{1}{8}$ + $\frac{1}{7}$ - $\frac{1}{9}$ + $\frac{1}{8}$ - $\frac{1}{10}$
= 1 + $\frac{1}{2}$ - $\frac{1}{9}$ - $\frac{1}{10}$
= $\frac{3}{2}$ - $\frac{1}{9}$- $\frac{1}{10}$
= $\frac{25}{18}$ -$\frac{1}{10}$
= $\frac{58}{45}$
=> B = $\frac{29}{45}$
