Question 1076394: calculate the surface area of a CD (compact disk) if the outer diameter is 12cm and the inner diameter is 2.5cm.
Answer by addingup(3677)   (Show Source):
You can put this solution on YOUR website! The outer diameter is 12. So the radius is 12/2 = 6
6^2 x Pi = 113.10
1.25^2*Pi = 4.91
113.10-4.91 = 108.19 this is the surface area of the CD.
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What do you know about Pi? Did you know that there is a Pi day? Pi Day is celebrated on March 14th (3/14) around the world because 3.14 are the first three digits of Pi, which is an irrational number and goes on forever, a researcher has calculated the 2,000,000,000,000,000th digit (two-quadrillionth digit) of the number, and that's not the end.
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But it has a very useful function. In your problem, when you do 6^2 (six squared) you literally have 4 squares. But 4 squares are too big for a circle, see picture I made you below. So how much of the squares do you need for the circle? Well, Pi of course, about 3.1416 parts of the 4 squares
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So to finish boring you, I said pi is an irrational number. What does that mean? It means that you cannot write it in the form of a RATIO so it's called IRRATIONAL. For example, 1.5 (one and one-half) can be written as 3/2. You think of this as a fraction, but you can call it a ratio, ratios can be written either 3:2 or 3/2.
Happy learning,
John
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