Math CentralQuandaries & Queries


Question from Mary, a student:

What is the diameter of a circle if the area is 132.7 square meters? I have tried the formula C=2 times pi radius and came up up 21.1. I also tried C = pi times d and came up with 42.26. The answer in the book says 13. How did they get this answer? Thank you for your help.

Hi Mary,

The expressions C = 2 π r and C = π d are expressions for the circumference C of a circle with diameter d meters and thus radius r = d/2 meters. You know the area of the circle is 132.7 square meters so you need to use an expression for the area. The area A of a circle is given by A = π r2 where r is the radius. Use this expression to find r2, take its square root to find r and then double r to find the diameter.


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