4. After sawing a 3-meter-long cylindrical wood with a bottom diameter of 2 decimeters into 4 sections, the surface area increased (37.68 square decimeters).

The cross-sectional area of Liugeng circle is 6*2π= 12π=37.68.

5. The radius of the bottom surface of the cylinder is doubled, the height is unchanged, and the volume is expanded to the original (4).

6. The side area of the cylinder is a square with a side length of 12.56 cm. The bottom area of this cylinder is (7.74) cm 2, and the volume of this cylinder is (24.30) cm 3.

2r = H2 *(2r+h)= 12.56h = 3. 14 r = 1.57

The bottom area s = π RR = 3.14 *1.57 *1.57 = 7.74.

Volume: v = π rrh = 3.14 *1.57 *1.57 *