how to :v

Em cũng thế :<

Em làm hết rồi nhưng.... cái đồng ý lại ko có :) Em định xóa bớt đi nhưng xóa hết mé rồi ;-;

\(A=\dfrac{1}{3}+\dfrac{1}{8}+\dfrac{1}{15}+\dfrac{1}{24}+\dfrac{1}{35}+\dfrac{1}{48}+\dfrac{1}{63}+\dfrac{1}{80}\\ A=\dfrac{1}{1\times3}+\dfrac{1}{2\times4}+\dfrac{1}{3\times5}+\dfrac{1}{4\times6}+\dfrac{1}{5\times7}+\dfrac{1}{6\times8}+\dfrac{1}{7\times9}+\dfrac{1}{8\times10}\\ A\times2=\left(\dfrac{2}{1\times3}+\dfrac{2}{3\times5}+\dfrac{2}{5\times7}+\dfrac{2}{7\times9}\right)+\left(\dfrac{2}{2\times4}+\dfrac{2}{4\times6}+\dfrac{2}{6\times8}+\dfrac{2}{8\times10}\right)\)\(A\times2=\left(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}\right)+\left(\dfrac{1}{2}+\dfrac{1}{4}-\dfrac{1}{4}+\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{10}\right)\\ A\times2=\dfrac{8}{9}+\dfrac{2}{5}\\ A\times2=\dfrac{58}{45}\\ A=\dfrac{29}{45}\)

lúc đầu câu 17 là think ạ