\(A=\dfrac{4}{19.21}+\dfrac{4}{21.23}+\dfrac{12}{23.29}+\dfrac{4}{29.31}\)
\(A=2\left(\dfrac{1}{19}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{23}+\dfrac{1}{23}-\dfrac{1}{29}+\dfrac{1}{29}-\dfrac{1}{31}\right)\)
\(A=2\left(\dfrac{1}{19}-\dfrac{1}{31}\right)\)
Cái này tự tính được,khuay r t lười đi lấy mt lắm
Theo đề ta có:
A=\(\dfrac{4}{19.21}+\dfrac{4}{21.23}+\dfrac{12}{23.29}+\dfrac{4}{29.31}\)
=> 2.(\(\dfrac{2}{19.21}+\dfrac{2}{21.23}+\dfrac{6}{23.29}+\dfrac{2}{29.31}\))
=> 2. \((\dfrac{2}{19.21}+\dfrac{2}{21.23}+\dfrac{2}{29.31})+\dfrac{6}{23.29}\)
=> 2. \(\left(\dfrac{1}{19}+\dfrac{1}{21}-\dfrac{1}{21}+\dfrac{1}{23}+......+\dfrac{1}{29}-\dfrac{1}{31}\right)\)
=> 2.( \(\dfrac{1}{19}-\dfrac{1}{31}\))
=> 2.( \(\dfrac{31}{589}-\dfrac{19}{589}\))
=> 2. \(\dfrac{12}{589}\)
=> \(\dfrac{24}{589}\)
Vậy A= \(\dfrac{24}{589}\)
