Mình trinh bày hơi 6677, bạn thông cảm.
Bài 4:
\(a,\dfrac{\dfrac{-6}{7}+\dfrac{6}{19}-\dfrac{6}{31}}{\dfrac{9}{7}-\dfrac{9}{19}+\dfrac{9}{31}}=\dfrac{\dfrac{-6}{7}-\dfrac{-6}{19}+\dfrac{-6}{31}}{\dfrac{9}{7}-\dfrac{9}{19}+\dfrac{9}{31}}=\dfrac{-6.\left(\dfrac{1}{7}-\dfrac{1}{19}+\dfrac{1}{31}\right)}{9\left(\dfrac{1}{7}-\dfrac{1}{19}+\dfrac{1}{31}\right)}=\dfrac{-6}{9}=\dfrac{-2}{3}\)
\(b,\dfrac{\dfrac{1}{9}-\dfrac{1}{7}-\dfrac{1}{11}}{\dfrac{4}{9}-\dfrac{4}{7}-\dfrac{4}{11}}+\dfrac{\dfrac{3}{5}-\dfrac{3}{25}-\dfrac{3}{125}-\dfrac{3}{625}}{\dfrac{4}{5}-\dfrac{4}{25}-\dfrac{4}{125}-\dfrac{4}{625}}\)
\(=\dfrac{\dfrac{1}{9}-\dfrac{1}{7}-\dfrac{1}{11}}{4.\left(\dfrac{1}{9}-\dfrac{1}{7}-\dfrac{1}{11}\right)}+\dfrac{3.\left(\dfrac{1}{5}-\dfrac{1}{25}-\dfrac{1}{125}-\dfrac{1}{625}\right)}{5.\left(\dfrac{1}{5}-\dfrac{1}{25}-\dfrac{1}{125}-\dfrac{1}{625}\right)}\)
\(=\dfrac{1}{4}+\dfrac{3}{5}\)
\(=\dfrac{5}{20}+\dfrac{12}{20}\)
\(=\dfrac{17}{20}\)