Let and let be the vertices of a regular -gon (in that order) with center . For and , let denote the centroid of the triangle Here the subscripts are taken modulo . If for relatively prime positive integers and , find .
Proposed by Yang Liu
Let and let be the vertices of a regular -gon (in that order) with center . For and , let denote the centroid of the triangle Here the subscripts are taken modulo . If for relatively prime positive integers and , find .
Proposed by Yang Liu
1. Convert the problem to the complex plane:
- Let , where .
- The vertices of the regular -gon can be represented as for .
2. Determine the centroid of the triangle:
- The centroid of a triangle with vertices is given by .
- For , the centroid is:
3. Generalize the centroid calculation:
- For , the centroid is:
- By induction, for , the centroid is:
4. Simplify the expression:
- Notice that the sum of all -th roots of unity is zero:
- Therefore, the sum is a subset of the -th roots of unity, and it sums to zero:
5. Calculate the magnitude:
- Since the sum is zero, we have:
- The magnitude of is:
6. Compare magnitudes:
- Initially, .
- Therefore, the ratio is:
- Since , we have:
7. **Find and :**
- Here, and , so .
The final answer is