Đáp án:
A=$\frac{1}{15}$ + $\frac{1}{20}$ + ... +$\frac{1}{120}$
=$\frac{2}{20}$ + $\frac{2}{30}$ + ... + $\frac{2}{240}$
=$\frac{2}{4.5}$ + $\frac{2}{5.6}$ + ... + $\frac{2}{15.16}$
=2. ($\frac{2}{4.5}$ + $\frac{2}{5.6}$ + ... + $\frac{2}{15.16}$)
=2. $\frac{1}{4}$ ( $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{7}$ + ... + $\frac{1}{15}$ - $\frac{1}{16}$ )
=2. ($\frac{1}{4}$ - $\frac{1}{16}$ )
=2. $\frac{3}{16}$
=$\frac{3}{8}$
chúc em học tốt