Question

Tính

A = \(\dfrac{3}{2.5}\)+\(\dfrac{4}{5.6}\)+\(\dfrac{7}{6.13}\)+\(\dfrac{11}{13.24}\)

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

Answer 1

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

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

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

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

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

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

Answer 2

A=3.97274066919