Problem:
Find the angle between the following two vectors:
$\left(
\begin{array}{c}
 -\frac{15}{4} \\
 \frac{3}{2} \\
 \frac{1}{4} \\
\end{array}
\right)$ and
$\left(
\begin{array}{c}
 -9 \\
 -\frac{13}{2} \\
 -\frac{15}{4} \\
\end{array}
\right)$.
Answer:
$\cos ^{-1}\left(\frac{369}{13 \sqrt{3406}}\right)$
{'source': 'AMPS', 'file': 'amps/mathematica/linear_algebra/vector_angles_and_projections/23945.txt'}