$\frac{1}{2 × 3}$ + $\frac{1}{3 × 4}$ + $\frac{1}{4 × 5}$ + $\frac{1}{5 × 6}$
Ta thấy : $\frac{1}{2 × 3}$ = $\frac{1}{2}$ - $\frac{1}{3}$
$\frac{1}{3 × 4}$ = $\frac{1}{3}$ - $\frac{1}{4}$
$\frac{1}{4 × 5}$ = $\frac{1}{4}$ - $\frac{1}{5}$
$\frac{1}{5 × 6}$ = $\frac{1}{5}$ - $\frac{1}{6}$
= $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + ... + $\frac{1}{5}$ - $\frac{1}{6}$
= $\frac{1}{2}$ - $\frac{1}{6}$
= $\frac{3}{6}$ - $\frac{1}{6}$
= $\frac{2}{6}$
= $\frac{1}{3}$.
