Đáp án:
A = $\frac{1}{10}$ + $\frac{1}{15}$ + $\frac{1}{21}$ + .... + $\frac{1}{55}$ + $\frac{1}{66}$
A = 2 ( $\frac{1}{20}$ + $\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$ + $\frac{1}{72}$ + $\frac{1}{90}$ + $\frac{1}{110}$ + $\frac{1}{132}$ )
A = 2 ( $\frac{1}{4.5}$ + $\frac{1}{5.6}$ + $\frac{1}{6.7}$ + $\frac{1}{7.8}$ + $\frac{1}{8.9}$ + $\frac{1}{9.10}$ + $\frac{1}{10.11}$ )
A = 2 [( $\frac{1}{4}$ - $\frac{1}{5}$ ) + ( $\frac{1}{5}$ - $\frac{1}{6}$ ) + ( $\frac{1}{6}$ - $\frac{1}{7}$ ) + ( $\frac{1}{7}$ - $\frac{1}{8}$ ) + ( $\frac{1}{8}$ - $\frac{1}{9}$ ) + ( $\frac{1}{9}$ - $\frac{1}{10}$ ) + ( $\frac{1}{10}$ - $\frac{1}{11}$ )]
A = 2 ( $\frac{1}{4}$ - $\frac{1}{12}$ )
⇒ A = $\frac{1}{3}$
