Step 1:

A right circular cone of radius r,height h, generated when the area bounded by the line OA(Where a is the point(h,r))

Step 2:

The x-axis and $x=h$ is rotated about the x-axis

Equation of OA is $\large\frac{y}{x}=\frac{r}{h}$$=>y=\large\frac{r}{h}$$x$

The volume of the cone$=\pi\int\limits_0^h y^2dx=\pi\int\limits_0^h \large\frac{r^2}{h^2}$$x^2dx$