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Algebra Level 5

α , β , γ ( 1 + α ) 2 1 α \large \displaystyle \sum_{\alpha,\beta,\gamma} \frac{(1+\alpha)^2}{1-\alpha}

Given that α , β \alpha,\beta and γ \gamma are the roots of x 3 x 1 = 0 x^3-x-1=0 , then find the value of above expression.


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The answer is -17.

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2 solutions

Aditya Sky
May 22, 2016

First of all, note that x = 1 x=1 is not a root to the equation x 3 x 1 = 0 x^{3}-x-1=0 , so, the value of given expression is evaluate-able.

Notice that ( 1 + α ) 2 1 α = ( 1 α ) 2 + 4 α 1 α = 1 α + 4 ( α 1 α ) \dfrac{(1+\alpha)^{2}}{1-\alpha}\,=\,\dfrac{(1-\alpha)^{2}+4\alpha}{1-\alpha}\,=\,1-\alpha+4\left(\dfrac{\alpha}{1-\alpha}\right) . Similarly, ( 1 + β ) 2 1 β = 1 β + 4 ( β 1 β ) \dfrac{(1+\beta)^{2}}{1-\beta}\,=\,1-\beta+4\left(\dfrac{\beta}{1-\beta}\right) and γ 2 1 γ = 1 γ + 4 ( γ 1 γ ) \dfrac{\gamma^{2}}{1-\gamma}\,=\,1-\gamma+4\left(\dfrac{\gamma}{1-\gamma}\right) .

So, cyc ( 1 + α ) 2 1 α = 3 ( α + β + γ ) + 4 ( α 1 α + β 1 β + γ 1 γ ) \sum_{\text{cyc}}\dfrac{(1+\alpha)^{2}}{1-\alpha}\,=\,3-(\alpha+\beta+\gamma)+4\left(\dfrac{\alpha}{1-\alpha}+\dfrac{\beta}{1-\beta}+\dfrac{\gamma}{1-\gamma}\right) .

Now, replacing x x by x 1 + x \dfrac{x}{1+x} in the equation x 3 x + 1 = 0 x^{3}-x+1\,=\,0 followed by appropriate simplifications gives x 3 + 5 x 2 + 4 x + 1 = 0 x^{3}+5x^{2}+4x+1\,=\,0 . Clearly, this equation has roots of the form α 1 α \dfrac{\alpha}{1-\alpha} . Now, Viete's Theorem, α 1 α + β 1 β + γ 1 γ = 5 \dfrac{\alpha}{1-\alpha}+\dfrac{\beta}{1-\beta}+\dfrac{\gamma}{1-\gamma}\,=\,-5 . So, cyc ( 1 + α ) 2 1 α = 3 0 + 4 ( 5 ) = 17 \sum_{\text{cyc}}\dfrac{(1+\alpha)^{2}}{1-\alpha}\,=\,3-0+4\cdot(-5)\,=\,\boxed{-17} .

Moderator note:

Since you're doing a substitution, can we directly use u = ( 1 + x ) 2 1 x u = \frac{ (1+x)^2}{1-x} ? Why, or why not?

Since you're doing a substitution, can we directly use u = ( 1 + x ) 2 1 x u = \frac{ (1+x)^2}{1-x} ? Why, or why not?

Calvin Lin Staff - 5 years ago
Hung Woei Neoh
May 22, 2016

n = { α , β , γ } ( 1 + n ) 2 1 n = ( 1 + α ) 2 1 α + ( 1 + β ) 2 1 β + ( 1 + γ ) 2 1 γ \displaystyle \sum_{n=\{\alpha,\beta,\gamma\}} \dfrac{(1+n)^2}{1-n}\\ =\dfrac{(1+\alpha)^2}{1-\alpha}+\dfrac{(1+\beta)^2}{1-\beta}+\dfrac{(1+\gamma)^2}{1-\gamma}

From Vieta's formula, we know that

α + β + γ = 0 α β + α γ + β γ = 1 α β γ = 1 \alpha + \beta + \gamma = 0\\ \alpha\beta+\alpha\gamma + \beta\gamma = -1\\ \alpha\beta\gamma = 1

Now, to simplify our calculations, we substitute α = a + 1 \alpha = a + 1 , β = b + 1 \beta = b+1 and γ = c + 1 \gamma = c+1 into the expression and the equations

The expression becomes:

( a + 2 ) 2 a + ( b + 2 ) 2 b + ( c + 2 ) 2 c = ( b c ( a 2 + 4 a + 4 ) + a c ( b 2 + 4 b + 4 ) + a b ( c 2 + 4 c + 4 ) a b c ) = ( a 2 b c + a b 2 c + a b c 2 + 12 a b c + 4 a b + 4 a c + 4 b c a b c ) = ( a b c ( a + b + c ) + 12 a b c + 4 ( a b + a c + b c ) a b c ) \dfrac{(a+2)^2}{-a} + \dfrac{(b+2)^2}{-b} + \dfrac{(c+2)^2}{-c}\\ =-\left(\dfrac{bc(a^2+4a+4) + ac(b^2 + 4b+4) + ab(c^2 + 4c + 4)}{abc}\right)\\ =-\left(\dfrac{a^2bc + ab^2c + abc^2 + 12abc + 4ab + 4ac + 4bc}{abc} \right)\\ =-\left(\dfrac{abc(a+b+c) + 12abc + 4(ab+ac+bc)}{abc}\right)

The equations become (simplify yourself, I'm too lazy to type out all the steps):

a + 1 + b + 1 + c + 1 = 0 a + b + c = 3 ( a + 1 ) ( b + 1 ) + ( a + 1 ) ( c + 1 ) + ( b + 1 ) ( c + 1 ) = 1 a b + a c + b c = 2 ( a + 1 ) ( b + 1 ) ( c + 1 ) = 1 a b c = 1 a + 1 + b + 1 + c + 1 = 0 \implies a+b+c = -3\\ (a+1)(b+1) + (a+1)(c+1) + (b+1)(c+1) = -1 \implies ab+ac+bc = 2\\ (a+1)(b+1)(c+1) = 1 \implies abc=1

Substitute these values into the expression to get the answer:

( a b c ( a + b + c ) + 12 a b c + 4 ( a b + a c + b c ) a b c ) = ( 1 ( 3 ) + 12 ( 1 ) + 4 ( 2 ) 1 ) = 17 -\left(\dfrac{abc(a+b+c) + 12abc + 4(ab+ac+bc)}{abc}\right)\\ =-\left(\dfrac{1(-3) + 12(1) + 4(2)}{1}\right)\\ =\boxed{-17}

First line should have some + signs right?

Calvin Lin Staff - 5 years ago

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Ah, typo, thanks

Hung Woei Neoh - 5 years ago

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