An algebra problem by Aly Ahmed

Algebra Level 2

If x 1 x_1 and x 2 x_2 are the roots of the equation x 2 8 x + 11 = 0 x^2-8x+11 = 0 , determine the value of

x 1 3 + x 1 2 + x 1 + x 2 3 + x 2 2 + x 2 x_1^3+x_1^2+x_1 + x_2^3+x_2^2+x_2


The answer is 298.

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2 solutions

Given that:

x 2 8 x + 11 = 0 x 2 = 8 x 11 x 3 = 8 x 2 11 x = 8 ( 8 x 11 ) 11 x = 53 x 88 x 3 + x 2 + x = 62 x 99 \begin{aligned} x^2 - 8x + 11 & = 0 \\ \implies x^2 & = 8x - 11 \\ x^3 & = 8x^2 - 11x \\ & = 8(8x-11) - 11x \\ & = 53x - 88 \\ \implies x^3 + x^2 + x & = 62x - 99 \end{aligned}

Since x 1 x_1 and x 2 x_2 are the roots of x 2 8 x + 11 x^2-8x + 11 , the sum:

S = x 1 3 + x 1 2 + x 1 + x 2 3 + x 2 2 + x 2 = 62 x 1 99 + 62 x 2 99 = 62 ( x 1 + x 2 ) 198 By Vieta’s formula x 1 + x 2 = 8 = 62 ( 8 ) 198 = 298 \begin{aligned} S & = x_1^3 + x_1^2 + x_1 + x_2^3 + x_2^2 + x_2 \\ & = 62x_1 - 99 + 62x_2 - 99 \\ & = 62 \blue{(x_1+x_2)} - 198 & \small \blue{\text{By Vieta's formula }x_1+x_2 = 8} \\ & = 62 \blue{(8)} - 198 \\ & = \boxed{298} \end{aligned}


Reference: Vieta's formula

Is this known as transformation of roots? It's quite brilliant!

Mahdi Raza - 11 months, 1 week ago

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Glad that you like it again.

Chew-Seong Cheong - 11 months, 1 week ago

No. That's like if f ( x ) = 0 f(x)=0 has roots a a and b b , then f ( 1 / x ) = 0 f(1/x) = 0 has roots 1 / a 1/a and b b .

Pi Han Goh - 11 months, 1 week ago

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  • Maybe... but I cross-verified whether it is and I think this method is known as transformation of roots.
  • And, the statement you said is under inversion of the transformation of roots. Whereas what is done is I think squaring in the transformation of roots. Here is the wiki (plus, I think you have a typo there for b b instead of 1 / b 1/b )
  • Do correct me if i am wrong... Thanks!

Mahdi Raza - 11 months, 1 week ago

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@Mahdi Raza Yup, you're right!

Pi Han Goh - 11 months, 1 week ago
Ved Pradhan
Jul 4, 2020

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