Tính giá trị biểu thức: 

- Hướng dẫn giải

$\begin{array}{l}D=\frac{3}{1.4}+\frac{6}{4.10}+\frac{9}{10.19}+\frac{12}{19.31}+\frac{15}{31.46}+\frac{18}{46.64}\\ =\frac{4-1}{1.4}+\frac{10-4}{4.10}+\frac{19-10}{10.19}+\frac{31-19}{19.31}+\frac{46-31}{31.46}+\frac{64-46}{46.64}\\ =1-\frac{1}{4}+\frac{1}{4}-\frac{1}{10}+\frac{1}{10}-\frac{1}{19}+\frac{1}{19}-\frac{1}{31}+\frac{1}{31}-\frac{1}{46}+\frac{1}{46}-\frac{1}{64}\\ =1-\frac{1}{64}\\ =\frac{63}{64}\end{array}$