\(1-\dfrac{1}{5}-\dfrac{2}{7}=\dfrac{35}{35}-\dfrac{7}{35}-\dfrac{10}{35}=\dfrac{18}{35}\)
\(=1-\left(\dfrac{1}{5}+\dfrac{2}{7}\right)\\ =1-\left(\dfrac{7}{35}+\dfrac{10}{35}\right)\\ =\dfrac{35}{35}-\dfrac{17}{35}=\dfrac{18}{35}\)
\(1-\dfrac{1}{5}-\dfrac{2}{7}\)
\(=\dfrac{4}{5}-\dfrac{2}{7}\)
\(=\dfrac{28}{35}-\dfrac{10}{35}\)
\(=\dfrac{18}{35}\)
=1−(1/5+2/7)=1−(7/35+10/35)=35/35−17/35=18/35