When a 12 cm long cylinder is cut 5 cm thick, the surface area of the cylinder is reduced by 31.4 square centimeters. How many square centimeters is the original surface area of the cylinder?

When a 12 cm long cylinder is cut 5 cm thick, the surface area of the cylinder is reduced by 31.4 square centimeters. How many square centimeters is the original surface area of the cylinder?

Bottom perimeter: 31.4 △ 5 = 6.28 (CM), bottom radius: 6.28 △ 3.14 △ 2 = 1 (CM), two bottom areas: 3.14 × 12 × 2 = 6.28 (cm 2), side area: 6.28 × 12 = 75.36 (cm 2), surface area: 75.36 + 6.28 = 81.64 (cm 2); answer: the original surface area of circular column is 81.64 cm 2

The expanded side area of a cylinder is square. It is known that its bottom perimeter is 8 square centimeters. The side area of the cylinder is square
The product is ()

Because C = H
So 8 * 8 = 64 (square centimeter)
A: the side area of this cylinder is 64 square centimeters

The height of a cylinder is 2 cm longer than the radius of its bottom, and the circumference of the expanded side view of the cylinder is 33.12 cm
What is the diameter? (3.14)

Let the radius be r cm and the height be (R + 2) cm
2×（2π×r+r+2）=33.12
6.28r+r+2=33.12÷2
7.28r=16.56-2
7.28r=14.56
r=2
A: the radius is 2 cm