Vieta's for the Cubic

Algebra Level 3

One of the roots of the equation 2 x 3 5 x 2 + 3 2x^3-5x^2+3 is 1 1 . The other 2 roots can be expressed as a ± b c \frac{a\pm{\sqrt{b}}}{c} . Find a + b + c a+b+c .

(Please add a elegant and thorough solution)


The answer is 40.

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1 solution

Chew-Seong Cheong
Dec 30, 2014

It is given that 1 1 is one of the roots of 2 x 3 5 x 2 + 3 2x^3-5x^2+3 . Let the other two roots be α \alpha and β \beta . Using Vieta's Formulas, we have:

( 1 ) ( α ) ( β ) = 3 2 β = 3 2 α (1)(\alpha)( \beta) = \dfrac {-3}{2} \quad \Rightarrow \beta = \dfrac {-3}{2\alpha}

We also know that:

1 + α + β = 5 2 α + β = 5 2 1 = 3 2 α 3 2 α = 3 2 1+\alpha+\beta = \dfrac {5}{2}\quad \Rightarrow \alpha + \beta = \dfrac {5}{2} - 1 = \dfrac {3}{2} \quad \Rightarrow \alpha - \dfrac {3}{2\alpha} = \dfrac {3}{2}

2 α 2 3 = 3 α 2 α 2 3 α 3 = 0 2\alpha^2 - 3 = 3\alpha \quad \Rightarrow 2\alpha^2 - 3\alpha - 3 = 0

α = ( 3 ) ± ( 3 ) 2 4 ( 2 ) ( 3 ) 2 ( 2 ) = 3 ± 33 4 = a ± b c \Rightarrow \alpha = \dfrac {-(-3) \pm \sqrt{(-3)^2-4(2)(-3)}}{2(2)} = \dfrac {3 \pm \sqrt{33}}{4} = \dfrac {a \pm \sqrt{b}}{c}

a + b + c = 3 + 33 + 4 = 40 \Rightarrow a+b+c = 3+33+4 = \boxed{40}

Given that we know that a root is 1, wouldn't it be easier to just find 2 x 3 5 x 2 + 3 x 1 = 2 x 2 3 x 3 \frac{ 2x^3 - 5x^2 + 3 } { x-1} = 2 x^2 - 3x - 3 ?

Calvin Lin Staff - 6 years, 5 months ago

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I did that actually. Then I noticed the question name is about Vieta

Chew-Seong Cheong - 6 years, 5 months ago

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Wow...........That was a big giveaway!

William Isoroku - 6 years, 5 months ago

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