SOLUTION: if the cost for materials to make a circular metal disk 3 cm in diameter is $.15, what would be the cost for materials to make a disk of the same thickness but 6 cm in diameter?

Question 330573: if the cost for materials to make a circular metal disk 3 cm in diameter is $.15, what would be the cost for materials to make a disk of the same thickness but 6 cm in diameter?
would the answer be $.45? Thanks.
Answer by Fombitz(32388) (Show Source): You can put this solution on YOUR website!
The assumption is that the cost of materials is based on the area of the disk (since the thickness is equivalent).
, where is total cost, is cost per unit area, is area.
The area of a circle is,

From the first example,

The area for the second disk is,

Since A2 is 4 times as much as A1, then the total cost will be 4 times as much.

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$0.60
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