AlgebraDifficulty 7.7National Olympiad, round 2Prove itHong Kong
Let a, b, c, d be roots of the equation x4+x+1=0. Let a5+2a+1, b5+2b+1, c5+2c+1, d5+2d+1 be roots of the equation x4+px3+qx2+rx+s=0. Find the value of p+2q+4r+8s.
Solution
The answer is 30. Firstly, since a4+a+1=0, we have a5+2a+1=a(a4+a+1)−a2+a+1=−a2+a+1. Let y=−x2+x+1. Then x=21±−4y+5. Now, 0=x4+x+1=(23−2y±−4y+5)2+21±−4y+5+1=22y2−8y+7±(3−2y)−4y+5+21±−4y+5+1=y2−4y+5±(2−y)−4y+5. This implies (y2−4y+5)2=(y−2)2(−4y+5), which is a degree 4 polynomial equation. Therefore, a5+2a+1, b5+2b+1, c5+2c+1 and d5+2d+1 are roots of the equation f(y)=(y2−4y+5)2−(y−2)2(−4y+5)=0. Note that f is monic. So it is exactly x4+px3+qx2+rx+s. Thus, p+2q+4r+8s=8f(21)−21=30.
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