α , β \alpha,\beta

Algebra Level 3

Let α \alpha and β \beta be the zeros of

x 2 3 x + 1 \large x^2-\sqrt{3}x+1

Find α 10 + β 10 \alpha^{10}+\beta^{10} .


The answer is 1.

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2 solutions

Relevant wiki: Vieta's Formula Problem Solving - Intermediate

Solution without solving for x x .

x 2 3 x + 1 = 0 x 2 = 3 x 1 x 3 = 3 x 2 x = 3 ( 3 x 1 ) x = 2 x 3 x 5 = ( 3 x 1 ) ( 2 x 3 x ) = 2 3 x 2 5 x + 3 = 2 3 ( 3 x 1 ) 5 x + 3 = x 3 x 10 = ( x 3 ) 2 = x 2 2 3 x + 3 = x 2 3 x + 1 2 3 + 2 Note that x 2 3 x + 1 = 0 = 2 3 x \begin{aligned} x^2 - \sqrt 3 x + 1 & = 0 \\ x^2 & = \sqrt 3 x - 1 \\ x^3 & = \sqrt 3 x^2 - x \\ & = \sqrt 3\left(\sqrt 3 x - 1\right) - x \\ & = 2x - \sqrt 3 \\ x^5 & = \left(\sqrt 3 x - 1\right)\left(2x - \sqrt 3 x\right) \\ & = 2\sqrt 3 x^2 - 5x + \sqrt 3 \\ & = 2\sqrt 3 \left(\sqrt 3 x - 1\right) - 5x + \sqrt 3 \\ & = x - \sqrt 3 \\ x^{10} & = \left(x - \sqrt 3 \right)^ 2 \\ & = x^2 - 2\sqrt 3 x + 3 \\ & = {\color{#3D99F6} x^2 - \sqrt 3 x + 1} - 2\sqrt 3 + 2 & \small \color{#3D99F6} \text{Note that } x^2 - \sqrt 3 x + 1 = 0 \\ & = 2 - \sqrt 3x \end{aligned}

Since α \alpha and β \beta are roots of x 2 3 x + 1 = 0 x^2 - \sqrt 3 x + 1 = 0 , then

α 10 = 2 3 α β 10 = 2 3 β α 10 + β 10 = 4 3 ( α + β ) By Vieta’s formula = 4 3 ( 3 ) = 1 \begin{aligned} \alpha^{10} & = 2 - \sqrt 3 \alpha \\ \beta^{10} & = 2 - \sqrt 3 \beta \\ \implies \alpha^{10}+ \beta^{10} & = 4 - \sqrt 3 ({\color{#3D99F6}\alpha + \beta}) & \small \color{#3D99F6} \text{By Vieta's formula} \\ & = 4 - \sqrt 3 ({\color{#3D99F6}\sqrt 3}) \\ & = \boxed{1} \end{aligned}

As always , amazingly done sir!

Rishu Jaar - 3 years, 7 months ago
Rishu Jaar
Nov 1, 2017

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