\(a,\dfrac{2}{3}+\dfrac{3}{4}.\left(-\dfrac{4}{9}\right)\)
\(=\dfrac{2}{3}+\left(-\dfrac{1}{3}\right)\)
\(=\dfrac{1}{3}\)
\(b,\left(\dfrac{3}{4}-0,2\right)\left(0,4-\dfrac{4}{5}\right)\)
\(=\left(\dfrac{3}{4}-\dfrac{1}{5}\right)\left(\dfrac{2}{5}-\dfrac{4}{5}\right)\)
\(=\dfrac{11}{20}.\dfrac{-2}{5}\)
\(=-\dfrac{11}{50}\)