Đáp án:
Chọn đáp án D. $\dfrac{19}{500}$
Giải thích các bước giải:
$B=\dfrac{1}{5.10}+\dfrac{1}{10.15} +\dfrac{1}{15.20} +\dfrac{1}{20.25} +...+\dfrac{1}{95.100}$
$=\dfrac{1}{5}.(\dfrac{5}{5.10}+\dfrac{5}{10.15} +\dfrac{5}{15.20} +\dfrac{5}{20.25} +...+\dfrac{5}{95.100})$
$=\dfrac{1}{5}.(\dfrac{1}{5}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{15}+...+\dfrac{1}{95}-\dfrac{1}{100})$
$=\dfrac{1}{5}.(\dfrac{1}{5}-\dfrac{1}{100})$
$=\dfrac{1}{5}.\dfrac{19}{100}$
$=\dfrac{19}{500}$