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Algebra Difficulty 3.8 AMC 10/12 Find the answer Italy

Let P(x)=x3+ax2+bx+cP(x) = x^{3} + a x^{2} + b x + c. Given that the sum of two of the roots of the polynomial equals zero, which of the following relations among the coefficients of P(x)P(x) is always true?

Pick one

Solution

Solution:

The answer is (B). Let x0x_{0}, x1x_{1} and x1-x_{1} be the roots of the polynomial; it must hold, for every xx, that (xx0)(xx1)(x+x1)=x3+ax2+bx+c(x - x_{0})(x - x_{1})(x + x_{1}) = x^{3} + a x^{2} + b x + c. Expanding the products and equating the coefficients of the terms of the same degree (principle of identity of polynomials), we get x0=a-x_{0} = a, x12=b-x_{1}^{2} = b, x12x0=cx_{1}^{2} x_{0} = c, from which ab=ca b = c.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.