For a natural number , consider the points on the unit circle . Show that the product of the distances of these points from is .
Problem 1535
Official solution
1. Let for . These points are the -th roots of unity excluding .
2. The -th roots of unity are the solutions to the equation . These roots can be written as , where .
3. The polynomial whose roots are the -th roots of unity is . This can be factored as:
4. We are interested in the product of the distances of the points from . This is equivalent to evaluating the product:
5. To find this product, consider the polynomial . We can write:
6. Evaluating at , we get:
7. However, we need to consider the derivative of at to find the product of the distances. The derivative is:
8. Evaluating the derivative at , we get:
9. The product of the distances from to each is given by the absolute value of the leading coefficient of the polynomial formed by the roots :
Therefore, the product of the distances of these points from is .
The final answer is