The following sum
$$\sqrt{9 - \left(\frac{3}{n}\right)^2} \cdot \frac{3}{n} + \sqrt{9 - \left(\frac{6}{n}\right)^2} \cdot \frac{3}{n} + \ldots + \sqrt{9 - \left(\frac{3 n}{n}\right)^2} \cdot \frac{3}{n}$$
is a right Riemann sum for the definite integral $$\int_0^b f(x)\, dx$$ where $b$ =
and $f(x)$ =

The limit of these Riemann sums as $n \to \infty$ is