Olympiad Maths Prep

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Problem 944

AIME late
Geometry Difficulty 5.9 Prove it

49. There are two watches of different sizes, both of which keep accurate time. On their faces, there exists a fixed point MM, such that the triangle formed by point MM and the tips of the second hands of the two watches is similar at any given moment.
(Ye Tingyu, Ren Yansong Middle School, Ruian City, Zhejiang, 325202)

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

Official solution

Proof: Construct a complex plane as shown, let the complex numbers corresponding to points P,QP, Q be:
p=r1ei(b1wτ),q=a+r2ej(θ2+w) \begin{array}{l} p=r_{1} e^{i\left(b_{1} \mid w \tau\right)}, \\ q=a+r_{2} e^{j\left(\theta_{2}+w\right)} \end{array}
(Where tt is a parameter)
Assume that a point MM exists, whose corresponding complex number is Q0Q_{0}, which is the point corresponding to t=0t=0. Then,
mpqp=mp4qnp4. \frac{m-p}{q-p}=\frac{m \quad p_{4}}{q_{n}-p_{4}} .
m=p+(pp0(p)(qp)(qq0)=r1ev(iH1+mer+r1ejθ1r1ei1r2eiq2 - [aejuc(r2ejw2r1ej/)]=r1er1r1ej1r2ejπ2a \begin{array}{l} \therefore m=p+\frac{\left(p-\frac{p_{0}}{(p)}(q-p)\right.}{\left(q-q_{0}\right)} \\ =r_{1} e^{v^{\left(i H_{1}\right.}+m e r}+\frac{r_{1} e^{j \theta_{1}}}{r_{1} e^{i \|_{1}}-r_{2} e^{i q_{2}}} \\ \text { - }\left[a \mid e^{j u c}\left(r_{2} e^{j w_{2}}-r_{1} e^{j /}\right)\right] \\ =\frac{r_{1} e^{r_{1}}}{r_{1} e^{j_{1}}-r_{2} e^{j \pi_{2}}} \cdot a \text {. } \\ \end{array}
(From this, we know that r1r2r_{1} \neq r_{2}, and the denominator is not zero)
It is independent of the parameter tt, i.e., point MM is a fixed point.
Therefore, the fixed point MM exists.
Note: If the two circles have a common point SS, and P0=Q0=SP_{0}=Q_{0}=S, then the conclusion does not hold. In some cases, similar triangles may degenerate into a set of collinear points.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.