I have the following question:
Find $\measuredangle C$ (in radians) and the area of the triangle if $a = 5, b =4$ and $c = 8.$
What i did is find angle $C$ and once i have $C$ the rest should be easy.
\begin{align*} c^2 &= a^2 +b^2 -2ab\,\cos(\measuredangle C)\\ 64 &= 25+16-40\,\cos(\measuredangle C)\\ 64 &=41-40\,\cos(\measuredangle C)\\ 64-41&=-40\,\cos(\measuredangle C)\\ -40\,\cos(\measuredangle C)&=23\\ \cos(\measuredangle C)&=\frac{23}{-40}\\ \measuredangle C&=\cos^{-1}\left(\frac{23}{-40}\right) \end{align*} but my answer is not correct the book says it should be $2.18$ rad.