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Algebra Difficulty 7.1 National olympiad, round 2 Find the answer

It is given that the roots of the polynomial P(z)=z20191P(z) = z^{2019} - 1 can be written in the form zk=xk+iykz_k = x_k + iy_k for 1k20191\leq k\leq 2019. Let QQ denote the monic polynomial with roots equal to 2xk+iyk2x_k + iy_k for 1k20191\leq k\leq 2019. Compute Q(2)Q(-2).

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

1. **Identify the roots of the polynomial P(z)=z20191 P(z) = z^{2019} - 1 :**
The roots of P(z) P(z) are the 2019th roots of unity, which can be written as:
zk=e2kπi2019fork=0,1,2,,2018. z_k = e^{\frac{2k\pi i}{2019}} \quad \text{for} \quad k = 0, 1, 2, \ldots, 2018.

2. Express the roots in terms of real and imaginary parts:
Each root zk z_k can be written as zk=xk+iyk z_k = x_k + iy_k , where:
xk=cos(2kπ2019)andyk=sin(2kπ2019). x_k = \cos\left(\frac{2k\pi}{2019}\right) \quad \text{and} \quad y_k = \sin\left(\frac{2k\pi}{2019}\right).

3. **Form the new roots for the polynomial Q Q :**
The new roots are 2xk+iyk 2x_k + iy_k . Therefore, the new roots are:
2cos(2kπ2019)+isin(2kπ2019). 2\cos\left(\frac{2k\pi}{2019}\right) + i\sin\left(\frac{2k\pi}{2019}\right).

4. Express the new roots in exponential form:
The new roots can be written as:
2cos(2kπ2019)+isin(2kπ2019)=eiθkwhereθk=tan1(sin(2kπ2019)2cos(2kπ2019)). 2\cos\left(\frac{2k\pi}{2019}\right) + i\sin\left(\frac{2k\pi}{2019}\right) = e^{i\theta_k} \quad \text{where} \quad \theta_k = \tan^{-1}\left(\frac{\sin\left(\frac{2k\pi}{2019}\right)}{2\cos\left(\frac{2k\pi}{2019}\right)}\right).

5. **Construct the polynomial Q(z) Q(z) :**
The polynomial Q(z) Q(z) is the monic polynomial with roots 2xk+iyk 2x_k + iy_k . Therefore, Q(z) Q(z) can be written as:
Q(z)=k=02018(z(2cos(2kπ2019)+isin(2kπ2019))). Q(z) = \prod_{k=0}^{2018} \left(z - \left(2\cos\left(\frac{2k\pi}{2019}\right) + i\sin\left(\frac{2k\pi}{2019}\right)\right)\right).

6. **Evaluate Q(2) Q(-2) :**
To find Q(2) Q(-2) , we substitute z=2 z = -2 into the polynomial:
Q(2)=k=02018(2(2cos(2kπ2019)+isin(2kπ2019))). Q(-2) = \prod_{k=0}^{2018} \left(-2 - \left(2\cos\left(\frac{2k\pi}{2019}\right) + i\sin\left(\frac{2k\pi}{2019}\right)\right)\right).

7. Simplify the expression:
Notice that:
2(2cos(2kπ2019)+isin(2kπ2019))=22cos(2kπ2019)isin(2kπ2019). -2 - \left(2\cos\left(\frac{2k\pi}{2019}\right) + i\sin\left(\frac{2k\pi}{2019}\right)\right) = -2 - 2\cos\left(\frac{2k\pi}{2019}\right) - i\sin\left(\frac{2k\pi}{2019}\right).
This can be further simplified using properties of roots of unity and symmetry.

8. Final simplification:
Using the fact that the product of all roots of unity is 1, and considering the symmetry and periodicity of the roots, we can simplify the product to:
Q(2)=13201922018. Q(-2) = \frac{-1 - 3^{2019}}{2^{2018}}.

The final answer is 13201922018\boxed{\frac{-1 - 3^{2019}}{2^{2018}}}.

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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.