Maths Olympiad Prep

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Geometry Difficulty 7.4 National olympiad, round 2 Find the answer

Is it possible to draw circles on the plane so that every line intersects at least one of them but no more than 100100 of them?

Solution

To determine whether it is possible to draw circles on the plane such that every line intersects at least one of them but no more than 100 of them, we will argue by contradiction.

1. Assume such circles exist:
Suppose we have a set of circles such that every line intersects at least one of them but no more than 100 of them. Let O O be a point outside all these circles.

2. **Apply inversion with center O O **:
Under inversion with center O O , each circle ki k_i (with center Oi O_i and radius ri r_i ) is mapped to another circle that does not pass through O O . Denote these images by ki=(Oi,ri) k_i' = (O_i', r_i') .

3. Properties of the new family of circles:
The new family of circles ki k_i' has the property that any line or circle through O O meets at least one of them, but no more than 100 of them.

4. Define distances and angles:
Let di=OOi d_i = |OO_i'| . The angle αi \alpha_i at which the circle ki k_i' is seen from O O satisfies:
αiridi \alpha_i \asymp \frac{r_i'}{d_i}
This means there exist constants c1 c_1 and c2 c_2 such that:
c1ridiαic2ridi c_1 \cdot \frac{r_i'}{d_i} \leq \alpha_i \leq c_2 \cdot \frac{r_i'}{d_i}

5. Convergence of the series:
The series i=1αi \sum_{i=1}^{\infty} \alpha_i converges. If it did not, we could find a line through O O that intersects more than 100 circles, which contradicts our assumption. Therefore, the series ridi \sum \frac{r_i'}{d_i} must also converge.

6. **Fix a small ε>0 \varepsilon > 0 **:
There exists a distance d>0 d > 0 such that the family K K of circles ki k_i' that intersect the disk centered at O O with radius d d satisfies:
kiKridi<ε \sum_{k_i' \in K} \frac{r_i'}{d_i} < \varepsilon

7. Rotate and shrink a circle:
Consider a circle k0 k_0 with diameter d d that passes through O O . Rotate k0 k_0 around O O and denote the rotated circle by k0(φ) k_0(\varphi) . The range of φ \varphi when the circle kiK k_i' \in K is swept by the circumference of k0(φ) k_0(\varphi) is:
Δφridi \Delta \varphi \asymp \frac{r_i'}{d_i}
According to the inequality above, when φ \varphi runs through [0,2π) [0, 2\pi) , there will exist a k0(φ) k_0(\varphi) that does not intersect any kiK k_i' \in K , leading to a contradiction.

8. Addressing the edge case:
The claim Δφridi \Delta \varphi \asymp \frac{r_i'}{d_i} may not hold when di d_i is close to d d . In such cases, it holds that Δφridi \Delta \varphi \asymp \sqrt{\frac{r_i'}{d_i}} , which is not useful. To fix this, we rotate k0 k_0 and shrink it simultaneously. Let Ak0 A \in k_0 such that OA OA is a diameter of k0 k_0 , and B B be inside k0 k_0 with OBA \angle OBA obtuse. For any XAB X \in AB with AOX=φ \angle AOX = \varphi , denote by k0(φ) k_0(\varphi) the circle with diameter OX OX . Now, Δφridi \Delta \varphi \asymp \frac{r_i'}{d_i} holds, and taking sufficiently small ε \varepsilon , the argument shows there exists some k0(φ) k_0(\varphi) that does not intersect any circle, leading to a contradiction.

Therefore, it is impossible to draw such circles.

\blacksquare

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