1. Use a rectangular length as the circumference of the cylinder bottom and a rectangular width as the height of the cylinder, so that the volume of the bucket cylinder is the largest.

Let the radius of the cylinder bottom be r, then 2πr= 12.56, and r=2.

So the cylindrical bottom area of the barrel is π r 2 = 12.56 square decimeter.

2. When the water level rises by 4 cm, the bottom of the glass jar is 1 cm, and the rising volume is the volume of the cone, so that the volume of the cone is 4 * π * 1 2 = 12.56 cubic centimeters.