Đáp án:
Giải thích các bước giải:
$A=\dfrac{2}{3}+\dfrac{2}{15}+\dfrac{2}{35}+\dfrac{2}{63}+\dfrac{2}{99}+\dfrac{2}{143}+\dfrac{2}{195}$
$ $
$=\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+\dfrac{2}{9.11}+\dfrac{2}{11.13}+\dfrac{2}{13.15}$
$ $
$=1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}$
$ $
$=1-\dfrac{1}{15}$
$ $
$=\dfrac{14}{15}$
$ $
$ $
$ $
$ $
$B=(6\dfrac{4}{9}+3\dfrac{7}{11})-4\dfrac{4}{9}$
$ $
$=6+\dfrac{4}{9}+\dfrac{40}{11}-4-\dfrac{4}{9}$
$ $
$=6-4+\dfrac{40}{11}$
$ $
$=2+\dfrac{40}{11}$
$ $
$=\dfrac{22}{11}+\dfrac{40}{11}$
$ $
$=\dfrac{62}{11}$
