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Algebra Difficulty 7.7 National Olympiad, round 2 Prove it Hong Kong

Let aa, bb, cc, dd be roots of the equation x4+x+1=0x^4 + x + 1 = 0. Let a5+2a+1a^5 + 2a + 1, b5+2b+1b^5 + 2b + 1, c5+2c+1c^5 + 2c + 1, d5+2d+1d^5 + 2d + 1 be roots of the equation x4+px3+qx2+rx+s=0x^4 + px^3 + qx^2 + rx + s = 0. Find the value of p+2q+4r+8sp + 2q + 4r + 8s.

Solution

The answer is 3030.
Firstly, since a4+a+1=0a^4 + a + 1 = 0, we have
a5+2a+1=a(a4+a+1)a2+a+1=a2+a+1. a^5 + 2a + 1 = a(a^4 + a + 1) - a^2 + a + 1 = -a^2 + a + 1.
Let y=x2+x+1y = -x^2 + x + 1. Then x=1±4y+52x = \frac{1 \pm \sqrt{-4y+5}}{2}. Now,
0=x4+x+1=(32y±4y+52)2+1±4y+52+1=2y28y+7±(32y)4y+52+1±4y+52+1=y24y+5±(2y)4y+5. \begin{aligned} 0 &= x^4 + x + 1 \\ &= \left( \frac{3 - 2y \pm \sqrt{-4y+5}}{2} \right)^2 + \frac{1 \pm \sqrt{-4y+5}}{2} + 1 \\ &= \frac{2y^2 - 8y + 7 \pm (3 - 2y)\sqrt{-4y+5}}{2} + \frac{1 \pm \sqrt{-4y+5}}{2} + 1 \\ &= y^2 - 4y + 5 \pm (2 - y)\sqrt{-4y+5}. \end{aligned}
This implies (y24y+5)2=(y2)2(4y+5)(y^2 - 4y + 5)^2 = (y - 2)^2(-4y + 5), which is a degree 44 polynomial equation. Therefore, a5+2a+1a^5 + 2a + 1, b5+2b+1b^5 + 2b + 1, c5+2c+1c^5 + 2c + 1 and d5+2d+1d^5 + 2d + 1 are roots of the equation
f(y)=(y24y+5)2(y2)2(4y+5)=0. f(y) = (y^2 - 4y + 5)^2 - (y - 2)^2(-4y + 5) = 0.
Note that ff is monic. So it is exactly x4+px3+qx2+rx+sx^4 + px^3 + qx^2 + rx + s. Thus,
p+2q+4r+8s=8f(12)12=30. p + 2q + 4r + 8s = 8f\left(\frac{1}{2}\right) - \frac{1}{2} = 30.

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