Let z1,z2,z3,z4 be the solutions to the equation x4+3x3+3x2+3x+1=0. Then ∣z1∣+∣z2∣+∣z3∣+∣z4∣ can be written as da+bc, where c is a square-free positive integer, and a,b,d are positive integers with gcd(a,b,d)=1. Compute 1000a+100b+10c+d.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Note that x=0 is clearly not a solution, so we can divide the equation by x2 to get (x2+2+x21)+3(x+x1)+1=0. Letting y=x+x1, we get that y2+3y+1=0, so y=x+x1=2−3±5. Since 2−3+5 has absolute value less than 2, the associated x are on the unit circle, and thus the two solutions for x in this case each have magnitude 1. For 2−3−5, the roots are negative reals that are reciprocals of each other. Thus, the sum of their absolute values is the absolute value of their sum, which is 23+5. Thus, the sum of the magnitudes of the four solutions are 1+1+23+5=27+5.
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