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AMC10 2022 B

AMC10 2022 B · Q22

AMC10 2022 B · Q22. It mainly tests Circle theorems, Ratios in geometry.

Let $S$ be the set of circles in the coordinate plane that are tangent to each of the three circles with equations $x^{2}+y^{2}=4$, $x^{2}+y^{2}=64$, and $(x-5)^{2}+y^{2}=3$. What is the sum of the areas of all circles in $S$?
设 $S$ 为坐标平面中与圆 $x^{2}+y^{2}=4$、$x^{2}+y^{2}=64$ 和 $(x-5)^{2}+y^{2}=3$ 各相切的圆的集合。$S$ 中所有圆面积的和是多少?
(A) 48\pi 48\pi
(B) 68\pi 68\pi
(C) 96\pi 96\pi
(D) 102\pi 102\pi
(E) 136\pi\qquad 136\pi\qquad
Answer
Correct choice: (E)
正确答案:(E)
Solution
The circles match up as follows: Case $1$ is brown, Case $2$ is blue, Case $3$ is green, and Case 4 is red. Let $x^2 + y^2 = 64$ be circle $O$, $x^2 + y^2 = 4$ be circle $P$, and $(x-5)^2 + y^2 = 3$ be circle $Q$. All the circles in S are internally tangent to circle $O$. There are four cases with two circles belonging to each: $*$ $P$ and $Q$ are internally tangent to $S$. $*$ $P$ and $Q$ are externally tangent to $S$. $*$ $P$ is externally and Circle $Q$ is internally tangent to $S$. $*$ $P$ is internally and Circle $Q$ is externally tangent to $S$. Consider Cases $1$ and $4$ together. Since circles $O$ and $P$ have the same center, the line connecting the center of $S$ and the center of $O$ will pass through the tangency point of both $S$ and $O$ and the tangency point of $S$ and $P$. This line will be the diameter of $S$ and have length $r_P + r_O = 10$. Therefore the radius of $S$ in these cases is $5$. Consider Cases $2$ and $3$ together. Similarly to Cases $1$ and $4$, the line connecting the center of $S$ to the center of $O$ will pass through the tangency points. This time, however, the diameter of $S$ will have length $r_P-r_O=6$. Therefore, the radius of $S$ in these cases is $3$. The set of circles $S$ consists of $8$ circles - $4$ of which have radius $5$ and $4$ of which have radius $3$. The total area of all circles in $S$ is $4(5^2\pi + 3^2\pi) = 136\pi \Rightarrow \boxed{\textbf{(E)}}$.
这些圆的匹配如下:情况 $1$ 是棕色,情况 $2$ 是蓝色,情况 $3$ 是绿色,情况 $4$ 是红色。 设 $x^2 + y^2 = 64$ 为圆 $O$,$x^2 + y^2 = 4$ 为圆 $P$,$(x-5)^2 + y^2 = 3$ 为圆 $Q$。 $S$ 中的所有圆都与圆 $O$ 内相切。 有四种情况,每种情况有两个圆: $*$ $P$ 和 $Q$ 与 $S$ 内相切。 $*$ $P$ 和 $Q$ 与 $S$ 外相切。 $*$ $P$ 与 $S$ 外相切,圆 $Q$ 与 $S$ 内相切。 $*$ $P$ 与 $S$ 内相切,圆 $Q$ 与 $S$ 外相切。 考虑情况 $1$ 和 $4$。由于圆 $O$ 和 $P$ 有相同的圆心,连接 $S$ 的圆心与 $O$ 的圆心的直线将通过 $S$ 与 $O$ 的切点以及 $S$ 与 $P$ 的切点。这条线将是 $S$ 的直径,长度为 $r_P + r_O = 10$。因此这些情况中 $S$ 的半径为 $5$。 考虑情况 $2$ 和 $3$。类似于情况 $1$ 和 $4$,连接 $S$ 的圆心与 $O$ 的圆心的直线将通过切点。这次,$S$ 的直径长度为 $r_P-r_O=6$。因此这些情况中 $S$ 的半径为 $3$。 集合 $S$ 由 $8$ 个圆组成——其中 $4$ 个半径为 $5$,$4$ 个半径为 $3$。 $S$ 中所有圆的总面积为 $4(5^2\pi + 3^2\pi) = 136\pi \Rightarrow \boxed{\textbf{(E)}}$。
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