An object moves $8$ cm in a straight [line](https://artofproblemsolving.com/wiki/index.php/Line) from $A$ to $B$, turns at an angle $\alpha$, measured in radians and chosen at random from the interval $(0,\pi)$, and moves $5$ cm in a straight line to $C$. What is the [probability](https://artofproblemsolving.com/wiki/index.php/Probability) that $AC < 7$?
$\mathrm{(A)}\ \frac{1}{6} \qquad\mathrm{(B)}\ \frac{1}{5} \qquad\mathrm{(C)}\ \frac{1}{4} \qquad\mathrm{(D)}\ \frac{1}{3} \qquad\mathrm{(E)}\ \frac{1}{2}$
By the [Law of Cosines](https://artofproblemsolving.com/wiki/index.php/Law_of_Cosines),
\begin{align*} AB^2 + BC^2 - 2 AB \cdot BC \cos \alpha = 89 - 80 \cos \alpha = AC^2 & \frac 12\\ \end{align*}
It follows that $0 < \alpha < \frac {\pi}3$, and the probability is $\frac{\pi/3}{\pi} = \frac 13 \Rightarrow \mathrm{(D)}$.