Đáp án:
Giải thích các bước giải:
Ta có:
$\frac{1}{6}$ < $\frac{1}{5}$; $\frac{1}{7}$ < $\frac{1}{5}$; $\frac{1}{9}$ < $\frac{1}{5}$; $\frac{1}{10}$ = $\frac{1}{10}$; $\frac{1}{11}$ < $\frac{1}{10}$; $\frac{1}{17}$ < $\frac{1}{10}$
⇔ $\frac{1}{5}$ + $\frac{1}{6}$ + $\frac{1}{7}$ +....+ $\frac{1}{17}$ < $\frac{1}{5}$ + $\frac{1}{5}$ +....+ $\frac{1}{5}$
⇔ D < $\frac{1}{5}$.5 + $\frac{1}{10}$.8
⇔ D < 1 + $\frac{4}{5}$ = $\frac{9}{5}$ < 2
⇔ D < 2 (đpcm)
