Use the fact that the area of a circle of radius r is r2 to find the

Use the fact that the area of a circle of radius r is πr2 to find the volume of a cone of height 1 that has radius r at a height r. Think of the cone as being built of a stack of little circular disks with some small thickness Δr.
There is often more than one way to divide up a region to find an area or volume.