\(B=81.\left(\dfrac{12-\dfrac{12}{7}-\dfrac{12}{289}-\dfrac{12}{85}}{4-\dfrac{4}{7}-\dfrac{4}{289}-\dfrac{4}{85}}:\dfrac{5+\dfrac{5}{13}+\dfrac{5}{169}+\dfrac{5}{91}}{6+\dfrac{6}{13}+\dfrac{6}{169}+\dfrac{6}{91}}\right)\)
\(\Rightarrow B=81.\left(\dfrac{12\left(1-\dfrac{1}{7}-\dfrac{1}{289}-\dfrac{1}{85}\right)}{4\left(1-\dfrac{1}{7}-\dfrac{1}{289}-\dfrac{1}{85}\right)}:\dfrac{5\left(1+\dfrac{1}{13}+\dfrac{1}{169}+\dfrac{1}{91}\right)}{6\left(1+\dfrac{1}{13}+\dfrac{1}{169}+\dfrac{1}{91}\right)}\right)\)
\(\Rightarrow B=81.\left(\dfrac{12}{4}:\dfrac{5}{6}\right)\)
\(\Rightarrow B=81.\left(3.\dfrac{6}{5}\right)\)
\(\Rightarrow B=81.\dfrac{18}{5}\\ \Rightarrow B=\dfrac{1458}{5}\)
ulatr cái này hôm trước trường mik thi HSG có bài y đúc như này luôn :>
