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Surface area of a cylinder


To derive the formula of the surface area of a cylinder, we will start by showing you how you can make a cylinder :





Start with a rectangle and two circles

cylinder-template-image




Then, fold the rectangle until you make an open cylinder with it. An open cylinder is a cylinder that has no bases. A good real life example of an open cylinder is a pipe used to flow water if you have seen one before



Rectangle-folded-to-make-a-cylinder-image




Next, using the two circles as bases for the cylinder, put one on top of the cylinder and put one beneath it.

Circles-and-cylinder-image




Of course, the two circles will have the exact same size or the same diameter as the circles obtained by folding the rectangle

You Finally, you end up with your cylinder!

cylinder-image




Now, what did we go through so much trouble? Well if you can make the cylinder with the rectangle and the two circles, you can use them to derive the surface area of the cylinder. Does that make sense?

The area of the two circles is straightforward. The area of one circle is pi × r2, so for two circles, you get 2 × pi × r2

To find the area of the rectangle is a little bit tricky and subtle!

Let us take a closer look at our rectangle again.



cylinder-template-image




Thus, the longest side or folded side of the rectangle must be equal to 2 × pi × r, which is the circumference of the circle

To get the area of the rectangle, multiply h by 2 × pi × r and that is equal to 2 × pi × r × h

Therefore, the total surface area of the cylinder, call it SA is:

SA = 2 × pi × r2  +   2 × pi × r × h

Example #1:


Find the surface area of a cylinder with a radius of 2 cm, and a height of 1 cm

SA = 2 × pi × r2  +   2 × pi × r × h

SA = 2 × 3.14 × 22  +   2 × 3.14 × 2 × 1

SA = 6.28 × 4  +   6.28 × 2

SA = 25.12 + 12.56

Surface area = 37.68 cm2


Example #2:


Find the surface area of a cylinder with a radius of 4 cm, and a height of 3 cm

SA = 2 × pi × r2  +   2 × pi × r × h

SA = 2 × 3.14 × 42  +   2 × 3.14 × 4 × 3

SA = 6.28 × 16  +   6.28 × 12

SA = 100.48 + 75.36

Surface area = 175.84 cm2








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