SOLUTION: The equation of the inner circle is x2 + y2 = 9. The radius of the outer circle is three times the radius of the inner circle. What is the equation of the outer circle

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Question 299600: The equation of the inner circle is x2 + y2 = 9. The radius of the outer circle is three times the radius of the inner circle. What is the equation of the outer circle
Answer by dabanfield(803) (Show Source):
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The equation of the inner circle is x2 + y2 = 9. The radius of the outer circle is three times the radius of the inner circle. What is the equation of the outer circle
The standard form of a circle with center at (h,k) and radius r is:
(x-h)^2 + (y-k)^2 = r^2
The inner circle's equation can be rewritten as:
(x-0)^2 + (y-0)^2 = 3^2 and so has center at (0,0) and radius 3.
Since the outer circle also has center at (0,0) but has radius 3*3 = 9 its equation is:
(x-0)^2 + (y-0)^2 = 9^2 or
x^2 + y^2 = 81