Play with roots

Algebra Level 4

( 1 + α 2 ) ( 1 + β 2 ) ( 1 + γ 2 ) ( 1 + δ 2 ) \large(1+\alpha^{2})(1+\beta^{2})(1+\gamma^{2})(1+\delta^{2})

If α , β , γ , δ \alpha,\beta,\gamma,\delta are the roots of the equation x 4 + 4 x 3 6 x 2 + 7 x 9 = 0 x^{4}+4x^{3}-6x^{2}+7x-9=0 , then find the value of the above expression.


The answer is 13.

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2 solutions

Rishabh Tripathi
Mar 6, 2016

We can write

( x α ) ( x β ) ( x γ ) ( x δ ) = x 4 + 4 x 3 6 x 2 + 7 x 9 (x-\alpha)(x-\beta)(x-\gamma)(x-\delta)=x^{4}+4x^{3}-6x^{2}+7x-9

putting i i and i -i in x x and multiplying the equations,

( i α ) ( i α ) ( i β ) ( i β ) ( i γ ) ( i γ ) ( i δ ) ( i δ ) = ( 2 + 3 i ) ( 2 3 i ) (i-\alpha)(-i-\alpha)(i-\beta)(-i-\beta)(i-\gamma)(-i-\gamma)(i-\delta)(-i-\delta)=(-2+3i)(-2-3i)

( 1 + α 2 ) ( 1 + β 2 ) ( 1 + γ 2 ) ( 1 + δ 2 ) = 13 (1+\alpha^{2})(1+\beta^{2})(1+\gamma^{2})(1+\delta^{2})=13

Shouldn't it be LaTeX: ( x α ) ( x β ) ( x γ ) ( x δ ) = x 4 + 4 x 3 6 x 2 + 7 x 9 (x-\alpha)(x-\beta)(x-\gamma)(x-\delta)=x^4+4x^3-6x^2+7x-9

Anik Mandal - 5 years, 3 months ago

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oh sorry updated it.. I think I was sleeping when I posted it.. lol

Rishabh Tripathi - 5 years, 3 months ago

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No problem..:p

Anik Mandal - 5 years, 3 months ago
Rishabh Jain
Mar 7, 2016

Equation can be written as:- x 4 6 x 2 9 = x ( 4 x 2 + 7 ) x^4-6x^2-9=-x(4x^2+7) ( x 4 6 x 2 9 ) 2 = x 2 ( 4 x 2 + 7 ) 2 ( 1 ) \implies (x^4-6x^2-9)^2=x^2(4x^2+7)^2\cdots (1) 1 + α 2 = x α = x 1 1+\alpha^2=x\implies \alpha=\sqrt{x-1} Since α \alpha is a root of the equation ( 1 ) (1) : ( ( x 1 ) 2 6 ( x 1 ) 9 ) = ( x 1 ) ( 4 ( x 1 ) + 7 ) 2 \therefore ((x-1)^2-6(x-1)-9)=(x-1)(4(x-1)+7)^2 ( ( x 1 ) 2 6 ( x 1 ) 9 ) ( x 1 ) ( 4 x + 3 ) 2 = 0 \implies ((x-1)^2-6(x-1)-9)-(x-1)(4x+3)^2=0 We have to find the product of roots of this equation since its roots are 1 + α 2 , 1 + β 2 , 1 + γ 2 , 1 + δ 2 1+\alpha^{2},1+\beta^{2},1+\gamma^{2},1+\delta^{2} which can be calculated by Vieta's formula and since leading coefficient is 1 1 we only have to find the constant term which is given by:- 1 + 36 + 81 + 12 108 18 + 9 = 13 1+36+81+12-108-18+9=\large\boxed{13}

This is a nice solution. Thanks for sharing!

Pi Han Goh - 5 years, 3 months ago

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Ya .... I thought that this approach also need to be shared.. :-)

Rishabh Jain - 5 years, 3 months ago

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