Đáp án:
Giải thích các bước giải:
$S=\frac{1}{2}+ \frac{1}{6}+ \frac{1}{12}+...+ \frac{1}{56}$
$= \frac{1}{1.2}+ \frac{1}{2.3}+ \frac{1}{3.4}+....+ \frac{1}{6.7} + \frac{1}{7.8}$
$= 1- \frac{1}{2}+ \frac{1}{2} - \frac{1}{3}+ \frac{1}{3}- \frac{1}{4}+....+ \frac{1}{6}- \frac{1}{7} + \frac{1}{7}- \frac{1}{8}$
$=1- \frac{1}{8}= \frac{7}{8}$
