An algebra problem by Akshat Sharda

Algebra Level 5

log 2 ( α 12 + β 12 + γ 12 + δ 12 ) \log_{2}(\alpha^{12}+\beta^{12}+\gamma^{12}+\delta^{12})

Given that α , β , γ \alpha,\beta,\gamma and δ \delta are the roots of the equation x 4 + 4 x + 4 = 0 x^4+4x+4=0 . Find the value of the expression above.


The answer is 9.

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1 solution

Chew-Seong Cheong
Nov 30, 2015

x 4 + 4 x + 4 = 0 x 4 = 4 ( x + 1 ) x 12 = 4 3 ( x + 1 ) 3 = 64 ( x 3 + 3 x 2 + 3 x + 1 ) α 12 + β 12 + γ 12 + δ 12 = 64 ( α 3 + β 3 + γ 3 + δ 3 + 3 ( α 2 + β 2 + γ 2 + δ 2 ) + 3 ( α + β + γ + δ ) + 4 ) \begin{aligned} x^4+4x+4 & = 0 \\ \Rightarrow x^4 & = -4(x+1) \\ x^{12} & = -4^3(x+1)^3 \\ & = -64(x^3+3x^2+3x+1) \\ \Rightarrow \alpha^{12} + \beta^{12} + \gamma^{12} + \delta^{12} & = -64\left(\alpha^3 + \beta^3 + \gamma^3 + \delta^3 +3(\alpha^2 + \beta^2 + \gamma^2 + \delta^2)+3(\alpha + \beta + \gamma + \delta) + 4\right) \end{aligned}

By Vieta's formulas, we have:

α + β + γ + δ = 0 α 2 + β 2 + γ 2 + δ 2 = ( α + β + γ + δ ) 2 2 ( α β + α γ + α δ + β γ + β δ + γ δ ) = 0 2 2 ( 0 ) = 0 α 3 + β 3 + γ 3 + δ 3 = ( α + β + γ + δ ) ( α 2 + β 2 + γ 2 + δ 2 ) ( α β + α γ + α δ + β γ + β δ + γ δ ) ( α + β + γ + δ ) + 3 ( α β γ + α β δ + α γ δ + β γ δ ) = ( 0 ) ( 0 ) ( 0 ) ( 0 ) + 3 ( 4 ) = 12 \begin{aligned}\alpha + \beta + \gamma + \delta & = 0 \\ \alpha^2 + \beta^2 + \gamma^2 + \delta^2 & = (\alpha + \beta + \gamma + \delta)^2 - 2(\alpha \beta + \alpha \gamma + \alpha \delta + \beta \gamma + \beta \delta + \gamma \delta) \\ & = 0^2 -2(0) = 0 \\ \alpha^3 + \beta^3 + \gamma^3 + \delta^3 & = (\alpha + \beta + \gamma + \delta)(\alpha^2 + \beta^2 + \gamma^2 + \delta^2) \\ & \quad - (\alpha \beta + \alpha \gamma + \alpha \delta + \beta \gamma + \beta \delta + \gamma \delta)(\alpha + \beta + \gamma + \delta) + 3(\alpha \beta \gamma + \alpha \beta \delta + \alpha \gamma \delta + \beta \gamma \delta) \\ & = (0)(0) -(0)(0) +3(-4) = -12 \end{aligned}

Therefore,

α 12 + β 12 + γ 12 + δ 12 = 64 ( α 3 + β 3 + γ 3 + δ 3 + 3 ( α 2 + β 2 + γ 2 + δ 2 ) + 3 ( α + β + γ + δ ) + 4 ) = 64 ( 12 + 3 ( 0 ) + 3 ( 0 ) + 4 ) = 64 ( 8 ) = 2 9 \begin{aligned} \alpha^{12} + \beta^{12} + \gamma^{12} + \delta^{12} & = -64\left(\alpha^3 + \beta^3 + \gamma^3 + \delta^3 +3(\alpha^2 + \beta^2 + \gamma^2 + \delta^2)+3(\alpha + \beta + \gamma + \delta) + 4\right) \\ & = -64 (-12+3(0)+3(0)+4) = -64(-8) = 2^9 \end{aligned}

log 2 ( α 12 + β 12 + γ 12 + δ 12 ) = 9 \Rightarrow \log_2 \left(\alpha^{12} + \beta^{12} + \gamma^{12} + \delta^{12}\right) = \boxed{9}

Exactly !!

Akshat Sharda - 5 years, 6 months ago

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Well i used newton sum...pretty long it was...Nice soln .

Mohit Gupta - 5 years, 6 months ago

Its level 5!

Aparna Kalbande - 5 years, 6 months ago

exactly, same way. one thing is that if s 1 = s 2 = . . . = s n = 0 s_1=s_2=...=s_n=0 , p 1 = p 2 = p 3 = . . . = p n = 0 p_1=p_2=p_3=...=p_n=0 . so we see that both s 1 = s 2 = 0 s_1=s_2=0 , so p 1 = p 2 = 0 p_1=p_2=0 and p 3 = 3 s 3 = 12 p_3=3s_3=-12 . doesn't take>1 minute to solve in head.

Aareyan Manzoor - 5 years, 6 months ago

Exactly Same Way.

Kushagra Sahni - 5 years, 5 months ago

Sir , how did you get the general formula of ( α 12 + β 12 + γ 12 + δ 12 ) (\alpha^{12}+\beta^{12}+\gamma^{12}+\delta^{12}) ?? Can we have a general formula for ( α n + β n + γ n + δ n ) (\alpha^{n}+\beta^{n}+\gamma^{n}+\delta^{n}) , where n is any positive integer ???

Chirayu Bhardwaj - 5 years, 2 months ago

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For α n + β n + γ n + δ n \alpha^{n}+\beta^{n}+\gamma^{n}+\delta^{n} in general, we can use Newton's sums method or Newton's identities. Read this wiki .

Chew-Seong Cheong - 5 years, 2 months ago

o man!! i did same but forget writing +4 so get 64*12.............................

Dhruv Joshi - 4 years, 3 months ago

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