Câu $2.5$
$B=\dfrac{1}{5}-\dfrac{3}{7}+\dfrac{5}{9}-\dfrac{2}{11}+\dfrac{7}{13}-\dfrac{9}{16}-\dfrac{7}{13}+\dfrac{2}{11}-\dfrac{5}{9}+\dfrac{3}{7}-\dfrac{1}{5}$
$B=(\dfrac{1}{5}-\dfrac{1}{5})-(\dfrac{3}{7}-\dfrac{3}{7})+(\dfrac{5}{9}-\dfrac{5}{9})-(\dfrac{2}{11}-\dfrac{2}{11})+(\dfrac{7}{13}-\dfrac{7}{13})-\dfrac{9}{16}$
$B=0-0+0-0+0-\dfrac{9}{16}$
$B=-\dfrac{9}{16}$
Câu $2.6$
$C=\dfrac{1}{100}-\dfrac{1}{100.99}-\dfrac{1}{99.98}-\dfrac{1}{98.97}-...-\dfrac{1}{3.2}-\dfrac{1}{2.1}$
$C=\dfrac{1}{100}-(\dfrac{1}{100.99}+\dfrac{1}{99.98}+\dfrac{1}{98.97}+...+\dfrac{1}{3.2}+\dfrac{1}{2.1})$
$C=\dfrac{1}{100}-(\dfrac{1}{99}-\dfrac{1}{100}+\dfrac{1}{98}-\dfrac{1}{99}+\dfrac{1}{97}-\dfrac{1}{98}+...+\dfrac{1}{2}-\dfrac{1}{3}+1-\dfrac{1}{2})$
$C=\dfrac{1}{100}-(-\dfrac{1}{100}+1)$
$C=\dfrac{1}{100}-(-\dfrac{1}{100}+\dfrac{100}{100})$
$C=\dfrac{1}{100}-\dfrac{99}{100}$
$C=-\dfrac{98}{100}$
$C=-\dfrac{49}{50}$
