\(đk:\left\{{}\begin{matrix}y\ne0\\y\ne\pm\end{matrix}\right.\\ =\dfrac{5}{2y\left(y+3\right)}-\dfrac{4y-3y^3}{y\left(y-3\right)\left(y+3\right)}-3\\ =\dfrac{5.\left(y-3\right)}{2y\left(y+3\right)\left(y-3\right)}-\dfrac{2\left(4y-3y^3\right)}{2y\left(y-3\right)\left(y+3\right)}-\dfrac{3.2y.\left(y^2-9\right)}{2y\left(y-3\right)\left(y+3\right)}\\ =\dfrac{5y-15-8y+6y^3-6y^3+54y}{2y\left(y^2-9\right)}\\ =\dfrac{51y-15}{2y\left(y^2-9\right)}\\ =\dfrac{3\left(17y-5\right)}{2y\left(y^2-9\right)}\)
\(c,\dfrac{5}{2y^2+6y}-\dfrac{4y-3y^3}{y^3-9y}-3\)
\(=\dfrac{5}{2y\left(y+3\right)}-\dfrac{y\left(4-3y^2\right)}{y\left(y^2-9\right)}-3\)
\(=\dfrac{5}{2y\left(y+3\right)}-\dfrac{y\left(4-3y^2\right)}{y\left(y-3\right)\left(y+3\right)}-3\)
\(=\dfrac{5\left(y-3\right)-2y\left(4-3y^2\right)-6y\left(y^2-9\right)}{2y\left(y-3\right)\left(y+3\right)}\)
\(=\dfrac{5y-15-8y+6y^3-6y^3+54y}{2y\left(y-3\right)\left(y+3\right)}\)
\(=\dfrac{51y-15}{2y\left(y-3\right)\left(y+3\right)}\)
\(=\dfrac{3\left(17y-5\right)}{2y\left(y^2-9\right)}\)
