a) \(\left(\dfrac{2}{3}\right)^3.\left(\dfrac{-3}{4}^2\right).\left(-1\right)^5\)
\(=\dfrac{8}{27}.\dfrac{9}{16}.\left(-1\right)\)
\(=\dfrac{1}{6}.\left(-1\right)\)
=\(\dfrac{-1}{6}\)
b) \(\left(\dfrac{2}{5}\right)^2.\left(\dfrac{-5}{12}\right)^2\)
\(=\dfrac{4}{25}.\dfrac{25}{144}\)
\(=\dfrac{1}{36}\)