\(a,\dfrac{90}{108}-\dfrac{40}{72}=\dfrac{5}{6}-\dfrac{5}{9}=\dfrac{30}{36}-\dfrac{20}{36}=\dfrac{10}{36}=\dfrac{5}{18}\)
\(b,\dfrac{59}{8}+\dfrac{21}{4}=\dfrac{59}{8}+\dfrac{42}{8}=\dfrac{101}{8}\)
\(c,\dfrac{25}{2}-\dfrac{18}{13}+\dfrac{5}{13}=\dfrac{325}{26}-\dfrac{26}{26}+\dfrac{10}{26}=\dfrac{309}{26}\)
\(d,\dfrac{17}{13}-\left(\dfrac{25}{100}+\dfrac{4}{13}\right)=\dfrac{17}{13}-\dfrac{29}{52}=\dfrac{3}{4}\)
a) \(\dfrac{90}{108}-\dfrac{40}{72}\)
\(=\dfrac{5}{6}-\dfrac{5}{6}\)
\(=0\)