2022 AMC 10B Problem 22

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Concepts:tangent circlescircle areacasework

Difficulty rating: 2390

22.

Let SS be the set of circles in the coordinate plane that are tangent to each of the three circles with equations x2+y2=4,x2+y2=64,(x5)2+y2=3.\begin{align*}x^{2}+y^{2}&=4,\\ x^{2}+y^{2}&=64, \\ (x-5)^{2}+y^{2}&=3.\end{align*} What is the sum of the areas of all circles in S?S?

 48π \ 48 \pi

 68π \ 68 \pi

 96π \ 96 \pi

 102π \ 102 \pi

 136π \ 136 \pi

Solution:

Let x2+y2=64x^2 + y^2 = 64 be circle O,O, x2+y2=4x^2 + y^2 = 4 be circle P,P, and (x5)2+y2=3(x - 5)^2 + y^2 = 3 be circle Q.Q.

First note that every circle, R,R, in SS is internally tangent to O.O. Then we case on the tangency of RR with PP and Q.Q.

Case 1:1: This corresponds to the pink circle. This is where PP and QQ are internally tangent to R.R.

Case 2:2: This corresponds to the bluish circle. This is where PP and QQ are externally tangent to R.R.

Case 3:3: This corresponds to the green circle. This is where PP is externally and QQ is internally tangent to R.R.

Case 4:4: This corresponds to the red circle. This is where PP is internally and QQ is externally tangent to R.R.

We can consider cases 11 and 44 together. Note that OO and PP have the same center. This means that the line connecting the center of RR and OO passes through the tangency point of both SS and OO and SS and P.P.

This line is the diameter of R,R, and it has length rP+rO=2+8=10. r_P + r_O = 2 + 8 = 10. Therefore, the radius of RR is 5.5.

Consider cases 22 and 33 together. Similarly to above, the line connecting the center of RR and OO will pass through the tangency points.

This time, however, the diameter of RR is rPrO=82=6. r_P - r_O = 8 - 2 = 6. This makes the radius of RR 3.3.

SS contains 88 circles: 44 of which have radius 55 and 44 of which have radius 33 (this is because we can flip all the circles in the diagram over the x-axis to get 44 more circles).

The total area of the circles in SS is therefore 4(52π+32π)=136π. 4 (5^2 \pi + 3^2 \pi) = 136 \pi. Thus, E is the correct answer.

Problem 22 in Other Years