Large numbers don't faze me!

Algebra Level 4

Let α \alpha and β \beta be the roots of the equation x 2 + x + 1 = 0 x^2 + x + 1 = 0 , find α 1027 + β 1027 \alpha^{1027} + \beta^{1027} .


The answer is -1.

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6 solutions

Chew-Seong Cheong
Dec 11, 2016

Relevant wiki: Vieta's Formula - Forming Quadratics

x 2 + x + 1 = 0 Multiply both sides by x x 3 + x 2 + x = 0 Add both sides by 1 x 3 + x 2 + x + 1 = 1 x 3 + 0 = 1 x 3 = 1 \begin{aligned} x^2 + x + 1 & = 0 & \small \color{#3D99F6} \text{Multiply both sides by }x \\ x^3 + x^2 + x & = 0 & \small \color{#3D99F6} \text{Add both sides by }1 \\ x^3 + {\color{#3D99F6}x^2 + x + 1} & = 1 \\ x^3 + {\color{#3D99F6}0} & = 1 \\ \implies x^3 & = 1 \end{aligned}

Since α \alpha and β \beta are roots of x 2 + x + 1 = 0 x^2+x+1 =0 , α 3 = β 3 = 1 \implies \alpha^3 = \beta^3 = 1 . Therefore,

α 1027 + β 1027 = α 342 × 3 + 1 + β 342 × 3 + 1 = α + β By Vieta’s formula = 1 \begin{aligned} \alpha^{1027} + \beta^{1027} & = \alpha^{342\times 3 + 1} + \beta^{342\times 3 + 1} \\ & = \alpha + \beta & \small \color{#3D99F6} \text{By Vieta's formula} \\ & = \boxed{-1} \end{aligned}

How exactly do you know α 3 = β 3 \alpha^3 = \beta^3 ? I understand the rest of the solution

Alex Li - 4 years, 6 months ago

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He showed that if x 2 + x + 1 = 0 x^2+x+1=0 ,then x 3 = 1 x^3=1 .Therefore,since α \alpha and β \beta satisfy x 2 + x + 1 = 0 x^2+x+1=0 ,hence α 3 = β 3 = 1 \alpha^3=\beta^3=1 .

Abdur Rehman Zahid - 4 years, 6 months ago

We note that α 2 + α + 1 = 0 α 3 = 1 \alpha^2+\alpha+1=0\implies \alpha^3 = 1 . Similarly for β \beta .

Chew-Seong Cheong - 4 years, 6 months ago

Question, 1 3 \sqrt[3]{-1} is imaginary right, not real ?

Jason Chrysoprase - 4 years, 6 months ago

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( 1 ) 3 = 1 ( 1 ) 3 3 = 1 3 1 = 1 3 (-1)^3 = -1 \implies \sqrt[3]{(-1)^3} = \sqrt[3]{-1} \implies -1 = \sqrt[3]{-1}

Chew-Seong Cheong - 4 years, 6 months ago

why is this level 5 ? is it that hard ?

Rohit Kumar - 4 years, 6 months ago

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idk myself, i think some level 4 up get confused

Jason Chrysoprase - 4 years, 5 months ago

Because of Vieta- Cardano formula α + β = 1 , α β = 1 α = 1 β \alpha + \beta = - 1, \space \alpha \cdot \beta = 1 \Rightarrow \alpha = \frac{1}{\beta} \Rightarrow α 1027 + β 1027 = 1 β 1027 + β 1027 = 1 + β 2054 β 1027 , ( 1 ) \alpha^{1027} + \beta^{1027} = \frac{1}{\beta^{1027}} + \beta^{1027} = \frac{ 1 + \beta^{2054}}{\beta^{1027}}, \space (1) Due to β 1025 ( β 2 + β + 1 ) = 0 β 1027 = β 1026 β 1025 = β 1024 \beta^{1025}\cdot (\beta^2 + \beta + 1) = 0 \Rightarrow \beta^{1027} = - \beta^{1026} - \beta^{1025} = \beta^{1024} , keeping on with this process we get β 1027 = β \beta^{1027} = \beta and β 2054 = β 2 \beta^{2054} = \beta^2 . Therefore, ( 1 ) = 1 + β 2054 β 1027 = 1 + β 2 β = β β = 1 (1) = \frac{ 1 + \beta^{2054}}{\beta^{1027}} = \frac{1 + \beta^2}{\beta} = \frac{-\beta}{\beta} = -1

Note.- β 0 \beta \neq 0

Note first that x 2 + x + 1 = x 3 1 x 1 x^{2} + x + 1 = \dfrac{x^{3} - 1}{x - 1} for x 1 x \ne 1 . Then since 1 1 is not a solution to x 2 + x + 1 = 0 x^{2} + x + 1 = 0 we know that the roots α , β \alpha, \beta of this quadratic are such that α 3 = 1 = β 3 \alpha^{3} = 1 = \beta^{3} .

Then since 1027 = 3 342 + 1 1027 = 3*342 + 1 we have that α 1027 = α \alpha^{1027} = \alpha and β 1027 = β \beta^{1027} = \beta , and so by Vieta's

α 1027 + β 1027 = α + β = 1 \alpha^{1027} + \beta^{1027} = \alpha + \beta = \boxed{-1} .

Satyam Tripathi
Jan 23, 2017

Roots are omega and omega square

Mr. Math
Dec 19, 2016

Alpha = w and beta = w^2 ... Cube roots of unity

Kris Hauchecorne
Dec 17, 2016

a = (-1 + i sqrt(3))/2 = exp(i 2/3 pi)

b = (-1 - i sqrt(3))/2 = exp(-i 2/3 pi)

2q = a^1027 + b^1027

q = cos(1027*2/3 pi) = -0,5

solution = 2q = -1

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