0, \ a \in (\frac{\pi}{2}, \pi)\\\\
\sin a = \sqrt{1 - \cos^2a}\\\\
\sin a = \sqrt{1 - (-0.8)^2}\\\\
\sin a = \sqrt{1 - 0.64}\\\\
\sin a = \sqrt{0.36}\\\\
\boxed{\sin a = 0.6}" alt="\sin a > 0, \ a \in (\frac{\pi}{2}, \pi)\\\\
\sin a = \sqrt{1 - \cos^2a}\\\\
\sin a = \sqrt{1 - (-0.8)^2}\\\\
\sin a = \sqrt{1 - 0.64}\\\\
\sin a = \sqrt{0.36}\\\\
\boxed{\sin a = 0.6}" align="absmiddle" class="latex-formula">