$(\dfrac{1}{1\times3}+\dfrac{1}{3\times5}+\dfrac{1}{5\times7}+\dfrac{1}{7\times9}+\dfrac{1}{9\times11})\times y=\dfrac{2}{3}$
$2\times(\dfrac{1}{1\times3}+\dfrac{1}{3\times5}+\dfrac{1}{5\times7}+\dfrac{1}{7\times9}+\dfrac{1}{9\times11})\times y=\dfrac{2}{3}\times2$
$\dfrac{2}{1\times3}+\dfrac{2}{3\times5}+\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+\dfrac{2}{9\times11})\times y=\dfrac{4}{3}$
$(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11})\times y=\dfrac{4}{3}$
$(1-\dfrac{1}{11})\times y=\dfrac{4}{3}$
$\dfrac{10}{11})\times y=\dfrac{4}{3}$
$y=\dfrac{4}{3}:\dfrac{10}{11}$
$y=\dfrac{22}{15}$
Vậy $y=\dfrac{22}{15}$
