Answer
$\frac{3\sqrt {10}}{10},-\frac{\sqrt {10}}{10},-3$
Work Step by Step
Given $cos(x)=-\frac{4}{5}, 180^\circ\lt x\lt 270^\circ$, we have $90^\circ\lt \frac{x}{2}\lt 135^\circ$ and
$sin\frac{x}{2}=\sqrt {\frac{1-cos(x)}{2}}=\sqrt {\frac{1+4/5}{2}}=\frac{3\sqrt {10}}{10}$,
$cos\frac{x}{2}=-\sqrt {\frac{1+cos(x)}{2}}=\sqrt {\frac{1-4/5}{2}}=-\frac{\sqrt {10}}{10}$ and
$tan\frac{x}{2}=\frac{sin\frac{x}{2}}{cos\frac{x}{2}}=-3$
