Find the length of the spiral $c(t) = (t \cos t, t \sin t)$ for $0 \le t \le 3 \pi$ to three decimal places. Hint: use the formula $$\int \sqrt{1 + t^2} \, dt = \frac{1}{2} t \sqrt{1 + t^2} + \frac{1}{2} \ln \left( t + \sqrt{1 + t^2} \right) + C$$