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Geometry Difficulty 6.8 National olympiad Find the answer

Let n=2188=37+1n = 2188 = 3^7+1 and let A0(0),A1(0),...,An1(0)A_0^{(0)}, A_1^{(0)}, ..., A_{n-1}^{(0)} be the vertices of a regular nn-gon (in that order) with center OO . For i=1,2,,7i = 1, 2, \dots, 7 and j=0,1,,n1j=0,1,\dots,n-1, let Aj(i)A_j^{(i)} denote the centroid of the triangle Aj(i1)Aj+37i(i1)Aj+237i(i1). \triangle A_j^{(i-1)} A_{j+3^{7-i}}^{(i-1)} A_{j+2 \cdot 3^{7-i}}^{(i-1)}. Here the subscripts are taken modulo nn. If OA2014(7)OA2014(0)=pq \frac{|OA_{2014}^{(7)}|}{|OA_{2014}^{(0)}|} = \frac{p}{q} for relatively prime positive integers pp and qq, find p+qp+q.

Proposed by Yang Liu

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Convert the problem to the complex plane:
- Let ω=cis(2πn)\omega = \operatorname{cis}\left(\frac{2\pi}{n}\right), where cis(θ)=cos(θ)+isin(θ)\operatorname{cis}(\theta) = \cos(\theta) + i\sin(\theta).
- The vertices of the regular nn-gon can be represented as Aj(0)=ωjA_j^{(0)} = \omega^j for j=0,1,,n1j = 0, 1, \ldots, n-1.

2. Determine the centroid of the triangle:
- The centroid of a triangle with vertices α,β,γ\alpha, \beta, \gamma is given by α+β+γ3\frac{\alpha + \beta + \gamma}{3}.
- For i=1i = 1, the centroid Aj(1)A_j^{(1)} is:
Aj(1)=ωj+ωj+36+ωj+2363 A_j^{(1)} = \frac{\omega^j + \omega^{j+3^6} + \omega^{j+2 \cdot 3^6}}{3}

3. Generalize the centroid calculation:
- For i=2i = 2, the centroid Aj(2)A_j^{(2)} is:
Aj(2)=ωj+ωj+35+ωj+235++ωj+8359 A_j^{(2)} = \frac{\omega^j + \omega^{j+3^5} + \omega^{j+2 \cdot 3^5} + \ldots + \omega^{j+8 \cdot 3^5}}{9}
- By induction, for i=7i = 7, the centroid Aj(7)A_j^{(7)} is:
Aj(7)=ωj+ωj+1+ωj+2++ωj+37137 A_j^{(7)} = \frac{\omega^j + \omega^{j+1} + \omega^{j+2} + \ldots + \omega^{j+3^7-1}}{3^7}

4. Simplify the expression:
- Notice that the sum of all nn-th roots of unity is zero:
ω0+ω1++ωn1=0 \omega^0 + \omega^1 + \ldots + \omega^{n-1} = 0
- Therefore, the sum ωj+ωj+1++ωj+371\omega^j + \omega^{j+1} + \ldots + \omega^{j+3^7-1} is a subset of the nn-th roots of unity, and it sums to zero:
ωj+ωj+1++ωj+371=0 \omega^j + \omega^{j+1} + \ldots + \omega^{j+3^7-1} = 0

5. Calculate the magnitude:
- Since the sum is zero, we have:
Aj(7)=0ωj137=ωj137 A_j^{(7)} = \frac{0 - \omega^{j-1}}{3^7} = -\frac{\omega^{j-1}}{3^7}
- The magnitude of Aj(7)A_j^{(7)} is:
Aj(7)=ωj137=137 |A_j^{(7)}| = \left| -\frac{\omega^{j-1}}{3^7} \right| = \frac{1}{3^7}

6. Compare magnitudes:
- Initially, OA2014(0)=1|OA_{2014}^{(0)}| = 1.
- Therefore, the ratio is:
OA2014(7)OA2014(0)=1371=137 \frac{|OA_{2014}^{(7)}|}{|OA_{2014}^{(0)}|} = \frac{\frac{1}{3^7}}{1} = \frac{1}{3^7}
- Since 37=21873^7 = 2187, we have:
137=12187 \frac{1}{3^7} = \frac{1}{2187}

7. **Find pp and qq:**
- Here, p=1p = 1 and q=2187q = 2187, so p+q=1+2187=2188p + q = 1 + 2187 = 2188.

The final answer is 2188\boxed{2188}

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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.