2015 AMC 12B Problem 24

Attempt Problem 24 of the 2015 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2015 AMC 12B solutions, or check the answer key.

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24.

Four circles, no two of which are congruent, have centers at A,A, B,B, C,C, and D,D, and points PP and QQ lie on all four circles. The radius of circle AA is 58\dfrac58 times the radius of circle B,B, and the radius of circle CC is 58\dfrac58 times the radius of circle D.D. Furthermore, AB=CD=39AB = CD = 39 and PQ=48.PQ = 48. Let RR be the midpoint of PQ.\overline{PQ}. What is AR+BR+CR+DR?AR + BR + CR + DR?

180180

184184

188188

192192

196196

Answer: D
Concepts:radical axisperpendicular bisectorPythagorean Theorem
Difficulty rating: 2560
Small Hint:

All of A,B,C,D,RA, B, C, D, R lie on the perpendicular bisector of PQ,PQ, and PR=24PR = 24

Big Hint:

With AR=yAR = y and radius ratio 5:8,5{:}8, use y2+242=25x2y^2 + 24^2 = 25x^2 and (39y)2+242=64x2(39 - y)^2 + 24^2 = 64x^2

Solution:

Since every center is equidistant from PP and Q,Q, all four centers and RR lie on the perpendicular bisector of PQ,PQ, with PR=24.PR=24. First consider two centers whose radii are in the ratio 5:85{:}8 and whose distance apart is 39.39. If RR lies between them, let y=ARy = AR and x=15x = \tfrac15 of circle AA’s radius. Then y2+242=25x2y^2 + 24^2 = 25x^2 and (39y)2+242=64x2.(39 - y)^2 + 24^2 = 64x^2. Subtracting gives x2=392y,x^2 = 39 - 2y, so y2+50y399=0y^2 + 50y - 399 = 0 and y=7.y=7. Here x=5,x=5, so the two center distances from RR are 77 and 32,32, and the radii are 2525 and 40.40.

If instead the two centers lie on the same side of R,R, their distances are ww and w+39.w+39. The analogous equations give w250w399=0,w^2-50w-399=0, hence w=57,w=57, and the distances are 5757 and 96.96. In this case the radii are 51535\sqrt{153} and 8153.8\sqrt{153}. Using the same placement for both pairs (A,B)(A,B) and (C,D)(C,D) would give two congruent circles of each radius, contrary to the hypothesis. Thus one pair uses each placement. Their distance sums are 7+32=397+32=39 and 57+96=153,57+96=153, so the requested total is 39+153=192.39+153=192.

Thus, the correct answer is D.

Problem 23#23
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