Bài 3:\(a,\dfrac{5}{13}.\dfrac{-6}{8}+\dfrac{5}{8}.\dfrac{-7}{13}+1\dfrac{5}{8}=\dfrac{5}{8}.\dfrac{-6}{13}+\dfrac{5}{8}.\dfrac{-7}{13}+\dfrac{13}{8}=\dfrac{5}{13}.\left(\dfrac{-6}{8}+\dfrac{-7}{8}\right)+\dfrac{13}{8}=\dfrac{5}{13}.\dfrac{-13}{8}+\dfrac{13}{8}=\dfrac{-5}{8}+\dfrac{13}{8}=1.\)
\(b,\dfrac{15.8+15.4}{12.3}=\dfrac{15.\left(8+4\right)}{12.3}=\dfrac{15.12}{12.3}=\dfrac{5.1}{1.1}=5.\)
Bài 4:
\(a,\dfrac{2}{5}+\left(\dfrac{-7}{9}+\dfrac{-2}{5}\right)=\dfrac{2}{5}+\dfrac{-7}{9}+\dfrac{-2}{5}=\dfrac{2}{5}+\dfrac{-2}{5}+\dfrac{-7}{9}=0+\dfrac{-7}{9}=\dfrac{-7}{9}.\)
\(b,\left(\dfrac{7}{4}-\dfrac{2}{9}\right)-\left(\dfrac{3}{4}-1\dfrac{7}{3}\right)=\dfrac{7}{4}-\dfrac{2}{9}-\dfrac{3}{4}+\dfrac{10}{3}=\dfrac{7}{4}-\dfrac{3}{4}-\dfrac{2}{9}+\dfrac{10}{3}=1-\dfrac{2}{9}+\dfrac{10}{3}=\dfrac{27}{27}-\dfrac{6}{27}+\dfrac{90}{27}=\dfrac{111}{27}.\)
