Đáp án:
   $\frac{1}{1}$ × 2 + $\frac{1}{2}$ × 3 + $\frac{1}{3}$ ×4 + $\frac{1}{4}$ × 5 + $\frac{1}{5}$ × 6
=$\frac{1}{1}$ - $\frac{1}{2}$ + $\frac{1}{2}$ - $\frac{1}{3}$ +  $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ 
= $\frac{1}{1}$ - $\frac{1}{6}$ 
= $\frac{5}{6}$ 
xin ctrlhn