2015 AMC 12B 第 24 题

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24.

四个圆两两不全等,圆心分别为 AABBCCDD,点 PPQQ 都在这四个圆上。圆 AA 的半径是圆 BB 半径的 58\dfrac58,圆 CC 的半径是圆 DD 半径的 58\dfrac58。此外,AB=CD=39AB = CD = 39PQ=48PQ = 48。令 RRPQ\overline{PQ} 的中点。AR+BR+CR+DRAR + BR + CR + DR 是多少?

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

答案:D
知识点:根轴垂直平分线勾股定理
难度评级:2560
解答:

因为每个圆心到 PPQ,Q, 等距,所以四个圆心和 RR 都位于 PQ,PQ, 的垂直平分线上,并且 PR=24.PR=24. 先考虑半径比为 5:85{:}8、圆心距离为 39.39. 的两个圆心。如果 RR 位于它们之间,令 y=ARy = AR,并令 x=15x = \tfrac15 乘以圆 AA 的半径。则 y2+242=25x2y^2 + 24^2 = 25x^2(39y)2+242=64x2.(39 - y)^2 + 24^2 = 64x^2. 相减得到 x2=392y,x^2 = 39 - 2y,所以 y2+50y399=0y^2 + 50y - 399 = 0,从而 y=7.y=7. 此时 x=5,x=5,两个圆心到 RR 的距离为 7732,32,半径为 252540.40.

如果两个圆心位于 R,R, 的同一侧,它们到该点的距离为 www+39.w+39. 类似的方程给出 w250w399=0,w^2-50w-399=0,所以 w=57,w=57,两个距离为 575796.96. 此时半径为 51535\sqrt{153}8153.8\sqrt{153}. 如果两对 (A,B)(A,B)(C,D)(C,D) 都采用同一种位置关系,就会出现两对同半径的全等圆,与题设矛盾。因此两对必须分别采用一种位置关系。它们的距离和分别为 7+32=397+32=3957+96=153,57+96=153,所以所求总和为 39+153=192.39+153=192.

因此,正确答案是 D

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.

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