Tính A=1/3+1/6+1/10+....+1/190

* Hướng dẫn giải

= $\frac{1}{6}$ + $\frac{1}{12}$ + $\frac{1}{120}$ +...+ $\frac{1}{380}$
= $\frac{1}{\left(2x3\right)}$ + $\frac{1}{\left(3x4\right)}$+ $\frac{1}{\left(4x5\right)}$+ ... + $\frac{1}{\left(19x20\right)}$
= $\frac{1}{2}$- $\frac{1}{3}$+$\frac{1}{3}$- $\frac{1}{4}$ +....+ $\frac{1}{19} -$ $\frac{1}{20}$
= $\frac{1}{2}$ - $\frac{1}{20}$ = $\frac{9}{20}$
Suy ra A = $\frac{9}{20}$ x 2 = $\frac{9}{10}$