bài 1:
câu a)
\(\dfrac{3}{5}+\dfrac{6}{11}+\dfrac{7}{13}+\dfrac{2}{5}+\dfrac{16}{11}+\dfrac{19}{13}\\ =\left(\dfrac{3}{5}+\dfrac{2}{5}\right)+\left(\dfrac{6}{11}+\dfrac{16}{11}\right)+\left(\dfrac{7}{13}+\dfrac{19}{13}\right)\\ =1+2+2=5\)
câu b)
\(\dfrac{5}{8}+\dfrac{5}{12}-\dfrac{1}{8}+\dfrac{1}{12}\\ =\left(\dfrac{5}{8}-\dfrac{1}{8}\right)+\left(\dfrac{5}{12}+\dfrac{1}{12}\right)\\ =\dfrac{1}{2}+\dfrac{1}{2}=1\)
câu c
\(\dfrac{3}{2}\cdot\dfrac{4}{7}+\dfrac{3}{7}\cdot\dfrac{3}{2}\\ =\dfrac{3}{2}\cdot\left(\dfrac{4}{7}+\dfrac{3}{7}\right)\\ =\dfrac{3}{2}\cdot1=\dfrac{3}{2}\)
câu d
\(\dfrac{17}{14}\cdot\dfrac{3}{7}+\dfrac{4}{7}\cdot\dfrac{17}{14}-\dfrac{17}{14}\\ =\dfrac{17}{14}\cdot\left(\dfrac{3}{7}+\dfrac{4}{7}-1\right)\\ =\dfrac{17}{14}\cdot0=0\)
bài 3"
câu a
\(\dfrac{2}{3}\cdot\dfrac{10}{21}\cdot\dfrac{3}{2}\\ =\left(\dfrac{2}{3}\cdot\dfrac{3}{2}\right)\cdot\dfrac{10}{21}\\ =1\cdot\dfrac{10}{21}=\dfrac{10}{21}\)
câu b
\(\dfrac{22}{5}\cdot\dfrac{1}{2}\cdot\dfrac{5}{22}\\ =\left(\dfrac{22}{5}\cdot\dfrac{5}{22}\right)\cdot\dfrac{1}{2}\\ =1\cdot\dfrac{1}{2}=\dfrac{1}{2}\)
câu c
\(\dfrac{7}{2}\cdot\dfrac{23}{81}\cdot\dfrac{2}{7}\\ =\left(\dfrac{7}{2}\cdot\dfrac{2}{7}\right)\cdot\dfrac{23}{81}\\ =1\cdot\dfrac{23}{81}=\dfrac{23}{81}\)
câu d
\(\dfrac{5}{17}\cdot\dfrac{21}{4}\cdot\dfrac{35}{15}\cdot\dfrac{1}{7}\\ =\left(\dfrac{5}{17}\cdot\dfrac{35}{15}\right)\cdot\left(\dfrac{21}{4}\cdot\dfrac{1}{7}\right)\\=\dfrac{35}{51}\cdot\dfrac{3}{4}=\dfrac{35}{68}\)
