Finding The Actual Roots Might Be Painful 1

Algebra Level 5

If α , β , γ \alpha, \beta, \gamma are roots to the equation x 3 x 1 = 0 x^3-x-1=0 , then evaluate the value of the below expression: 1 + α 1 α + 1 + β 1 β + 1 + γ 1 γ \frac{1+\alpha}{1-\alpha} + \frac{1+\beta}{1-\beta} + \frac{1+\gamma}{1-\gamma}


The answer is -7.

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4 solutions

Muhammad Maulana
Feb 7, 2016

Considering the equation A X 3 + B X 2 + C X + D = 0 AX^{3}+BX^{2}+CX+D=0 given that A=1, B=0, C=-1, D=-1 . \text{A=1, B=0, C=-1, D=-1}. Using a vieta's theorem: α + β + γ = B A = 0 \alpha + \beta + \gamma= \frac {-B}{A}=0 α β + β γ + α γ = C A = 1 \alpha\beta + \beta\gamma + \alpha\gamma= \frac {C}{A}=-1 α β γ = D A = 1 \alpha\beta\gamma= \frac {-D}{A}=1 . Then: 1 + α 1 α + 1 + β 1 β + 1 + γ 1 γ \frac{1+\alpha}{1-\alpha} + \frac{1+\beta}{1-\beta} + \frac{1+\gamma}{1-\gamma} ( 1 + α ) ( 1 β ) ( 1 γ ) + ( 1 + β ) ( 1 α ) ( 1 γ ) + ( 1 + γ ) ( 1 α ) ( 1 β ) ( 1 α ) ( 1 β ) ( 1 γ ) \Rightarrow \frac{(1+\alpha)(1-\beta)(1-\gamma) + (1+\beta)(1-\alpha)(1-\gamma) + (1+\gamma)(1-\alpha)(1-\beta)}{(1-\alpha)(1-\beta)(1-\gamma)} 3 ( α + β + γ ) ( α β + β γ + α γ ) + 3 ( α β γ ) 1 ( α + β + γ ) + ( α β + β γ + α γ ) ( α β γ ) \Rightarrow \frac{3-(\alpha+\beta+\gamma) - (\alpha\beta + \beta\gamma + \alpha\gamma) + 3(\alpha\beta\gamma)}{1-(\alpha+\beta+\gamma) + (\alpha\beta + \beta\gamma + \alpha\gamma)-(\alpha\beta\gamma)} Substitued: 3 0 ( 1 ) + 3 ( 1 ) ( 1 0 + ( 1 ) 1 ) = 7 \Rightarrow \frac{3-0-(-1)+3(1)}{(1-0+(-1)-1)} =\boxed{-7}

Rohit Ner
Feb 8, 2016

1 + x = x 3 x 3 1 = x ( x 1 ) ( x 2 + x + 1 ) = x 1 x = x x 2 + 1 + x = x x 2 + x 3 = x x 2 ( 1 + x ) = 1 x 4 1 + x 1 x = x 7 x 6 = x 2 + 2 x + 1 x 7 = x 3 + 2 x 2 + x 1 + x 1 x = x 7 = ( x 3 + 2 x 2 + x ) = [ α 3 + β 3 + γ 3 + 2 ( α 2 + β 2 + γ 2 ) + α + β + γ ] = [ 3 α β γ + 2 ( 2 ( α β + β γ + γ α ) ) ] = ( 3 + 4 ) = 7 \begin{aligned} 1+x&={x}^3\\{x}^3-1 &=x\\\left(x-1\right)\left({x}^2+x+1\right)&=x\\1-x &=\frac{-x}{{x}^2+1+x}=\frac{-x}{{x}^2+{x}^3}=\frac{-x}{{x}^2\left(1+x\right)}=-\frac{1}{{x}^4}\\\frac{1+x}{1-x}&=-{x}^7\\{x}^6&={x}^2+2x+1\\{x}^7&={x}^3+2{x}^2+x\\\sum \frac{1+x}{1-x}&=-\sum {x}^7=-\sum \left({x}^3+2{x}^2+x\right) \\&=-\left[{\alpha}^3+{\beta}^3+{\gamma}^3+2\left({\alpha}^2+{\beta}^2+{\gamma}^2\right)+\alpha+\beta+\gamma\right]\\&=-\left[3\alpha\beta\gamma+2\left(-2\left(\alpha\beta+\beta\gamma+\gamma\alpha\right)\right)\right]\\&=-\left(3+4\right)\\&\huge\color{#3D99F6}{=\boxed{-7}}\end{aligned}

Beautiful solution!

Vandit Kumar - 3 years, 4 months ago
Budi Utomo
Feb 14, 2016

Punyak Johri
Feb 7, 2016

replace x by (x-1)/(x+1) and use vieta's to find sum of roots.

Exactly what I did. It worked!!

Hosam Hajjir - 5 years, 4 months ago

Yup, the roots are symmetrical, so we can do so.

Chetan Pandey - 5 years, 4 months ago

That was nice and neat .

Raven Herd - 5 years, 2 months ago

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