SOLUTION: Homework questions: The area of a circle is found by the equation A=pie*r^2. If the area A of a certain circle is 81pie square centimeters, find its radius r. I need to know how to

Algebra ->  Algebra  -> Circles -> SOLUTION: Homework questions: The area of a circle is found by the equation A=pie*r^2. If the area A of a certain circle is 81pie square centimeters, find its radius r. I need to know how to      Log On

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Question 113209: Homework questions: The area of a circle is found by the equation A=pie*r^2. If the area A of a certain circle is 81pie square centimeters, find its radius r. I need to know how to figure this to be able to get the answer.
Found 2 solutions by Earlsdon, solver91311:
Answer by Earlsdon(6103) About Me  (Show Source):
You can put this solution on YOUR website!
A+=+%28pi%29r%5E2 Substitute A = 81
81+=+%28pi%29r%5E2 Divide both sides by %28pi%29
81%2F%28pi%29+=+r%5E2 Take the square root of both sides.
9%2Fsqrt%28pi%29+=+r

Answer by solver91311(12126) About Me  (Show Source):
You can put this solution on YOUR website!
A=pi%2Ar%5E2 and A=81%2Api%2Acm%5E2

Since both right-hand expressions are equal to A, we can say that they are equal to each other,

pi%2Ar%5E2=81%2Api%2Acm%5E2

r%5E2=81cm%5E2, divide both sides by pi
r=sqrt%2881cm%5E2%29, take the square root of both sides.
In general, when you take the square root, you need to consider both the positive and negative square root. But in this case, you can exclude the negative square root because we are trying to find the length of a radius.

r=9cm

Hope that helps,
John