Using Vieta's Formula?

Algebra Level 4

2016 x 3 + 2015 x 2 + 2014 x + 2016 = 0 \large 2016x^{3}+2015x^{2}+2014x+2016=0

Let α \alpha , β \beta and γ \gamma be the roots of the equation above .

Find the value of :

2016 ( α 3 + β 3 + γ 3 ) + 2015 ( α 2 + β 2 + γ 2 ) + 2014 ( α + β + γ ) 2016 ( α 3 β 3 γ 3 ) + 2015 ( α 2 β 2 γ 2 ) + 2014 ( α β γ ) \dfrac{2016(\alpha^3+\beta^3+\gamma^3)+2015(\alpha^2+\beta^2+\gamma^2)+2014(\alpha+\beta+\gamma)}{2016(\alpha^3-\beta^3-\gamma^3)+2015(\alpha^2-\beta^2-\gamma^2)+2014(\alpha-\beta-\gamma)}


The answer is -3.

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3 solutions

Rishabh Jain
Jul 15, 2016

Relevant wiki: Vieta's Formula - Higher Degrees

Since α , β , γ \alpha,\beta,\gamma are roots if the given equation, hence:

2016 α 3 + 2015 α 2 + 2014 α = 2016... ( 1 ) 2016\alpha^{3}+2015\alpha^{2}+2014\alpha=-2016...(1)

2016 β 3 + 2015 β 2 + 2014 β = 2016... ( 2 ) 2016\beta^{3}+2015\beta^{2}+2014\beta=-2016...(2)

2016 γ 3 + 2015 γ 2 + 2014 γ = 2016... ( 3 ) 2016\gamma^{3}+2015\gamma^{2}+2014\gamma=-2016...(3)

LHS of ( 1 ) + ( 2 ) + ( 3 ) (1)+(2)+(3) gives the numerator while LHS of ( 1 ) ( 2 ) ( 3 ) (1)-(2)-(3) gives the denominator , hence answer is:

( 2016 + 2016 + 2016 ) ( 2016 2016 2016 ) = 3 \large \dfrac{-(2016+2016+2016)}{-(2016-2016-2016)}=\boxed{-3}

Nice solution! :)

A Former Brilliant Member - 4 years, 11 months ago

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Thanks... :-P

Rishabh Jain - 4 years, 11 months ago
Chew-Seong Cheong
Jul 15, 2016

Relevant wiki: Vieta's Formula - Higher Degrees

Since 2016 x 3 + 2015 x 2 + 2014 x + 2016 = 0 2016x^{3}+2015x^{2}+2014x+2016 = 0 2016 x 3 + 2015 x 2 + 2014 x = 2016 \implies 2016x^{3}+2015x^{2}+2014x = -2016 . And since α \alpha , β \beta and γ \gamma are the roots of the equation, then we have:

{ 2016 α 3 + 2015 α 2 + 2014 α = 2016 2016 β 3 + 2015 β 2 + 2014 β = 2016 2016 γ 3 + 2015 γ 2 + 2014 γ = 2016 \begin{cases} 2016 \alpha^{3}+2015\alpha^{2}+2014\alpha = -2016 \\ 2016 \beta^{3}+2015\beta^{2}+2014\beta = -2016 \\ 2016 \gamma^{3}+2015\gamma^{2}+2014\gamma = -2016 \end{cases}

Therefore,

2016 ( α 3 + β 3 + γ 3 ) + 2015 ( α 2 + β 2 + γ 2 ) + 2014 ( α + β + γ ) 2016 ( α 3 β 3 γ 3 ) + 2015 ( α 2 β 2 γ 2 ) + 2014 ( α β γ ) = 2016 2016 2016 2016 + 2016 + 2016 = 3 \dfrac{2016(\alpha^3+\beta^3+\gamma^3)+2015(\alpha^2+\beta^2+\gamma^2)+2014(\alpha+\beta+\gamma)}{2016(\alpha^3-\beta^3-\gamma^3)+2015(\alpha^2-\beta^2-\gamma^2)+2014(\alpha-\beta-\gamma)} = \frac {-2016-2016-2016}{-2016+2016+2016} = \boxed{-3}

Nice one sir

Amirul Syariz - 4 years, 11 months ago
Tommy Li
Jul 15, 2016

{ 2016 α 3 + 2015 α 2 + 2014 α + 2016 = 0 2016 β 3 + 2015 β 2 + 2014 β + 2016 = 0 2016 γ 3 + 2015 γ 2 + 2014 γ + 2016 = 0 \begin{cases} 2016\alpha^{3}+2015\alpha^{2}+2014\alpha+2016=0 \\ 2016\beta^{3}+2015\beta^{2}+2014\beta+2016=0 \\ 2016\gamma^{3}+2015\gamma^{2}+2014\gamma+2016=0 \end{cases}

{ 2016 α 3 + 2015 α 2 + 2014 α = 2016 2016 β 3 + 2015 β 2 + 2014 β = 2016 2016 γ 3 + 2015 γ 2 + 2014 γ = 2016 \begin{cases} 2016\alpha^{3}+2015\alpha^{2}+2014\alpha=-2016 \\ 2016\beta^{3}+2015\beta^{2}+2014\beta=-2016 \\ 2016\gamma^{3}+2015\gamma^{2}+2014\gamma=-2016 \end{cases}

2016 ( α 3 + β 3 + γ 3 ) + 2015 ( α 2 + β 2 + γ 2 ) + 2014 ( α + β + γ ) 2016 ( α 3 β 3 γ 3 ) + 2015 ( α 2 β 2 γ 2 ) + 2014 ( α β γ ) \dfrac{2016(\alpha^3+\beta^3+\gamma^3)+2015(\alpha^2+\beta^2+\gamma^2)+2014(\alpha+\beta+\gamma)}{2016(\alpha^3-\beta^3-\gamma^3)+2015(\alpha^2-\beta^2-\gamma^2)+2014(\alpha-\beta-\gamma)}

= ( 2016 ) × 3 ( 2016 ) ( 2016 ) × 2 =\dfrac{(-2016)\times3}{(-2016)-(-2016)\times2}

= 6048 2016 = - \dfrac{6048}{2016}

= 3 =-3

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