Vieta's might come in handy (Polynomials Sprint question)

Algebra Level 2

The roots of the equation

6 x 3 3 x 2 + 2 x 1 = 0 , 6x^3 - 3x^2 +2x - 1 = 0,

are α , β \alpha, \beta and γ \gamma . Find the value of

1 + α + β + γ + α β + β γ + γ α + α β γ . 1 + \alpha + \beta + \gamma + \alpha \beta + \beta \gamma + \gamma \alpha + \alpha \beta \gamma.


The answer is 2.

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4 solutions

Daniel Liu
Jun 24, 2014

Clever solution:

Note that 6 x 3 3 x 2 + 2 x 1 = 6 ( x α ) ( x β ) ( x γ ) 6x^3-3x^2+2x-1=6(x-\alpha)(x-\beta)(x-\gamma)

Also, 1 + α + β + γ + α β + β γ + γ α + α β γ = ( α + 1 ) ( β + 1 ) ( γ + 1 ) 1+\alpha+\beta+\gamma+\alpha\beta+\beta\gamma+\gamma\alpha+\alpha\beta\gamma=(\alpha+1)(\beta+1)(\gamma+1)

Now plug in x = 1 x=-1 in our first equation to get that 6 ( 1 α ) ( 1 β ) ( 1 γ ) = 6 ( 1 ) 3 3 ( 1 ) 2 + 2 ( 1 ) 1 = 12 6(-1-\alpha)(-1-\beta)(-1-\gamma)=6(-1)^3-3(-1)^2+2(-1)-1=-12 or ( 1 + α ) ( 1 + β ) ( 1 + γ ) = 2 -(1+\alpha)(1+\beta)(1+\gamma)=-2

Thus, ( 1 + α ) ( 1 + β ) ( 1 + γ ) = 2 (1+\alpha)(1+\beta)(1+\gamma)=\boxed{2}

Creative. But I think it's faster just to add up all the Vieta's equations and get 2.

Colin Tang - 6 years, 11 months ago

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Yea, that's what I did at first. I found this solution in hindsight.

Daniel Liu - 6 years, 11 months ago

Same method!

Swapnil Das - 6 years ago
Joanne Lee
Jun 24, 2014

6 x 3 3 x 2 + 2 x 1 = 0 x 3 1 2 x 2 + 1 3 x 1 6 = 0 6x^3-3x^2+2x-1=0 \implies x^3-\frac{1}{2}x^2+\frac{1}{3}x-\frac{1}{6}=0

( x α ) ( x β ) ( x γ ) = x 3 ( α + β + γ ) x 2 + ( α β + β γ + α γ ) x ( α β γ ) (x-\alpha)(x-\beta)(x-\gamma)=x^3-(\alpha+\beta+\gamma)x^2+(\alpha \beta + \beta \gamma + \alpha \gamma)x-(\alpha \beta \gamma)

1 + α + β + γ + α β + β γ + α γ + α β γ = 1 + 1 2 + 1 3 + 1 6 = 2 1+\alpha+\beta+\gamma+\alpha \beta + \beta \gamma + \alpha \gamma+\alpha \beta \gamma = 1 + \frac{1}{2} +\frac{1}{3}+\frac{1}{6}= \boxed{2}

Anand Raj
Jun 24, 2014

1 + ( Sum of roots) + (Sum of roots two at a time) + (Products of roots).......

And we get the results......... 1 + (1/2) + (1/3) + (1/6)

Zakaria Sellami
Jun 25, 2014

6 x 3 3 x 2 + 2 x 1 = 3 x 2 ( 2 x 1 ) + ( 2 x 1 ) = ( 2 x 1 ) ( 3 x 2 + 1 ) 6x^3-3x^2+2x-1=3x^2(2x-1)+(2x-1)=(2x-1)(3x^2+1)
Now, it's easy to find the roots of this polynomial in C \mathbb{C} that are: α = 1 / 2 , β = i / 3 \alpha=1/2 ,\hspace{1cm} \beta = i/\sqrt{3} \hspace{1cm} and γ = i / 3 \hspace{1cm} \gamma= -i/\sqrt{3}
After calculating, we find that the answer is 2 2 .

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