Đáp án:
D = $\frac{25}{258}$
Giải thích các bước giải:
D = $\frac{4}{8.13}$ + $\frac{4}{13.18}$ + $\frac{4}{18.23}$ + ..... + $\frac{4}{253.258}$
→ $\frac{5}{4}$D = $\frac{5}{8.13}$ + $\frac{5}{13.18}$ + $\frac{5}{18.23}$ + ..... + $\frac{5}{253.258}$
→ $\frac{5}{4}$D = $\frac{1}{8}$ - $\frac{1}{13}$ + $\frac{1}{13}$ - $\frac{1}{18}$ + $\frac{1}{18}$ - $\frac{1}{23}$ + ..... + $\frac{1}{253}$ - $\frac{1}{258}$
→ $\frac{5}{4}$D = $\frac{1}{8}$ - $\frac{1}{258}$
→ $\frac{5}{4}$D = $\frac{125}{1032}$
→ D = $\frac{25}{258}$
