Maths Olympiad Prep

Track / Stage 7 / 135 of 300 #1535 of 1964

Problem 1535

National olympiad second round; IMO P1/P4
Geometry Difficulty 7.2 Prove it

For a natural number n>1n>1 , consider the n1n-1 points on the unit circle e2πikn (k=1,2,...,n1)e^{\frac{2\pi ik}{n}}\ (k=1,2,...,n-1) . Show that the product of the distances of these points from 11 is nn.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

1. Let ωk=e2iπk/n\omega_k = e^{2 i \pi k/n} for k=1,2,,n1k = 1, 2, \ldots, n-1. These points are the nn-th roots of unity excluding 11.

2. The nn-th roots of unity are the solutions to the equation xn=1x^n = 1. These roots can be written as 1,ω1,ω2,,ωn11, \omega_1, \omega_2, \ldots, \omega_{n-1}, where ωk=e2iπk/n\omega_k = e^{2 i \pi k/n}.

3. The polynomial whose roots are the nn-th roots of unity is xn1x^n - 1. This can be factored as:
xn1=(x1)(xω1)(xω2)(xωn1). x^n - 1 = (x - 1)(x - \omega_1)(x - \omega_2) \cdots (x - \omega_{n-1}).

4. We are interested in the product of the distances of the points ωk\omega_k from 11. This is equivalent to evaluating the product:
k=1n11ωk. \prod_{k=1}^{n-1} |1 - \omega_k|.

5. To find this product, consider the polynomial P(x)=xn1P(x) = x^n - 1. We can write:
P(x)=(x1)(xω1)(xω2)(xωn1). P(x) = (x - 1)(x - \omega_1)(x - \omega_2) \cdots (x - \omega_{n-1}).

6. Evaluating P(x)P(x) at x=1x = 1, we get:
P(1)=1n1=0. P(1) = 1^n - 1 = 0.

7. However, we need to consider the derivative of P(x)P(x) at x=1x = 1 to find the product of the distances. The derivative P(x)P'(x) is:
P(x)=nxn1. P'(x) = n x^{n-1}.

8. Evaluating the derivative at x=1x = 1, we get:
P(1)=n1n1=n. P'(1) = n \cdot 1^{n-1} = n.

9. The product of the distances from 11 to each ωk\omega_k is given by the absolute value of the leading coefficient of the polynomial formed by the roots ωk\omega_k:
k=1n11ωk=P(1)=n. \prod_{k=1}^{n-1} |1 - \omega_k| = |P'(1)| = n.

Therefore, the product of the distances of these points from 11 is nn.

The final answer is n\boxed{n}

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