Find the limit of the rational function $($a$)$ as $x$ and $($b$)$ as $x-.$ Write $$ or $-$ where appropriate.

$f(x)=\frac{4x^{8}6x^2-1}{2x^7-4x9}$

a$.\underset{x}{lim}(\frac{4x^{8}6x^2-1}{2x^7-4x9})=$

$($Simplify your answer.$)$

b$.\underset{x-}{lim}(\frac{4x^{8}6x^2-1}{2x^7-4x9})=$

$($Simplify your answer.$)$