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geometry senior
Problem
Let be the set of circles in the coordinate plane that are tangent to each of the three circles with equations , , and . What is the sum of the areas of all circles in ?
(A)
(B)
(C)
(D)
(E)
Solution
The circles match up as follows: Case is brown, Case is blue, Case is green, and Case 4 is red. Let be circle , be circle , and be circle . All the circles in S are internally tangent to circle . There are four cases with two circles belonging to each: and are internally tangent to . and are externally tangent to . is externally and Circle is internally tangent to . is internally and Circle is externally tangent to . Consider Cases and together. Since circles and have the same center, the line connecting the center of and the center of will pass through the tangency point of both and and the tangency point of and . This line will be the diameter of and have length . Therefore the radius of in these cases is . Consider Cases and together. Similarly to Cases and , the line connecting the center of to the center of will pass through the tangency points. This time, however, the diameter of will have length . Therefore, the radius of in these cases is . The set of circles consists of circles - of which have radius and of which have radius . The total area of all circles in is .
Final answer
E