Example 346

Problem: What is the area of the circle shown in Figure 3.6? Solution:

If we were assigned to paint a water storage tank, we must know the surface area of the walls of the tank to determine how much paint is required. In this case, we need to know the area of a circular or cylindrical tank. To determine the surface area of the tank, we need to visualize the cylindrical walls as a rectangle wrapped around a circular base. The area of a rectangle is found by multiplying the length by the width; in the case of a cylinder, the width of the rectangle is the height of the wall, and the length of the rectangle is the distance around the circle (circumference).

Thus, the area (A) of the side wall of a circular tank is found by multiplying the circumference of the base (C = n x D) times the height of the wall (H):

A = n x 20 ft x 25 ft = 3.14 x 20 ft x 25 ft = 1570 ft2

To determine the amount of paint needed, remember to add the surface area of the top of the tank, which is 314 ft2. Thus, the amount of paint needed must cover 1570 ft2 + 314 ft2 = 1884 ft2. If the tank floor should be painted, add another 314 ft2.

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