Mk làm bị thiếu cái này ms đúng.

A= \(\dfrac{2}{2.5}+\dfrac{2}{5.8}+\dfrac{2}{8.11}+...+\dfrac{2}{92.95}+\dfrac{2}{95.98}\)

= \(2.\dfrac{1}{3}.\left(\dfrac{1}{2.5}+\dfrac{1}{5.8}+\dfrac{1}{8.11}+...+\dfrac{1}{92.95}+\dfrac{1}{95.98}\right)\)

= \(\dfrac{2}{3}.\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+...+\dfrac{1}{92}-\dfrac{1}{95}+\dfrac{1}{95}-\dfrac{1}{98}\right)\)

= \(\dfrac{2}{3}.\left(\dfrac{1}{2}-\dfrac{1}{98}\right)\)

= \(\dfrac{2}{3}.\dfrac{24}{49}\)

= \(\dfrac{16}{49}\)

A= \(\dfrac{2}{2.5}+\dfrac{2}{5.8}+\dfrac{2}{8.11}+.....+\dfrac{2}{92.95}+\dfrac{2}{95.98}\)

= \(2.\left(\dfrac{1}{2.5}+\dfrac{1}{5.8}+\dfrac{1}{8.11}+....+\dfrac{1}{92.95}+\dfrac{1}{95.98}\right)\)

= \(2.\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+....+\dfrac{1}{92}-\dfrac{1}{95}+\dfrac{1}{95}-\dfrac{1}{98}\right)\)

= \(2.\left(\dfrac{1}{2}-\dfrac{1}{98}\right)\)

= \(2.\dfrac{24}{49}\)

= \(\dfrac{48}{49}\)

/x/-\(\dfrac{3}{4}=\dfrac{5}{3}\)

\(\Leftrightarrow\) /x/ = \(\dfrac{5}{3}+\dfrac{3}{4}\)

\(\Leftrightarrow\) /x/ = \(\dfrac{29}{12}\)

TH1: x =\(\dfrac{29}{12}\)

TH2: x = -\(\dfrac{29}{12}\)

Vậy x = \(\dfrac{29}{12}hoặc\) x = -\(\dfrac{29}{12}\)

B = \(\dfrac{2}{3.4}+\dfrac{2}{4.9}+\dfrac{2}{5.6}+\dfrac{2}{6.7}+\dfrac{2}{7.8}\)

= \(2.\left(\dfrac{1}{3.4}+\dfrac{1}{4.9}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}\right)\)

= \(2.\left(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{9}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}\right)\)

= \(2.\left(\dfrac{1}{3}-\dfrac{1}{9}-\dfrac{1}{8}\right)\)

= \(2.\dfrac{7}{72}\)

= \(\dfrac{7}{36}\)