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2003 AMC 10A Problem 18

Problem 18 of 25IntermediateAlgebra

What is the sum of the reciprocals of the roots of the equation 20032004x+1+1x=0?\dfrac{2003}{2004}x + 1 + \dfrac{1}{x} = 0?

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Solution

Let a=20032004.a = \dfrac{2003}{2004}. Multiplying the equation by xx gives ax2+x+1=0.ax^2 + x + 1 = 0. If the roots are rr and s,s, then by Vieta’s formulas r+s=−1ar + s = -\dfrac{1}{a} and rs=1a.rs = \dfrac{1}{a}. The sum of reciprocals is 1r+1s=r+srs=−1a1a=−1.\dfrac{1}{r} + \dfrac{1}{s} = \dfrac{r + s}{rs} = \dfrac{-\frac{1}{a}}{\frac{1}{a}} = -1. Thus, the correct answer is B.
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Tagged: Vieta’s Formulas · quadratic

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