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\(\dfrac{5}{3}+\left(\dfrac{-2}{7}\right)-\left(\dfrac{-1}{2}\right)\)
= \(\dfrac{5}{3}-\dfrac{2}{7}+\dfrac{1}{2}\)
= \(\dfrac{70}{42}-\dfrac{12}{42}+\dfrac{21}{42}\)
= \(\dfrac{79}{42}\)
\(\dfrac{5}{3}+\dfrac{-2}{7}-\dfrac{-1}{2}=\dfrac{98}{42}-\dfrac{12}{42}+\dfrac{21}{42}=\dfrac{107}{42}\)
Ta có :
\(A=\frac{1}{3}-\frac{3}{4}-\left(-\frac{3}{5}\right)+\frac{1}{72}-\frac{2}{9}-\frac{1}{36}+\frac{1}{15}\)
\(\Rightarrow A=\frac{5}{15}-\frac{54}{72}+\frac{9}{15}+\frac{1}{72}-\frac{16}{72}-\frac{1}{72}+\frac{1}{15}\)
\(\Rightarrow A=\left(\frac{5}{15}+\frac{9}{15}+\frac{1}{15}\right)+\left(-\frac{54}{72}+\frac{1}{72}-\frac{16}{72}-\frac{2}{72}\right)\)
\(\Rightarrow A=1-\frac{71}{72}=\frac{1}{72}\)
\(\dfrac{3}{7}\cdot\dfrac{9}{26}-\dfrac{1}{14}\cdot\dfrac{1}{13}\)
\(=\dfrac{3\cdot9-1}{14\cdot13}=\dfrac{26}{14\cdot13}=\dfrac{2}{14}=\dfrac{1}{7}\)