Easy one .

Algebra Level 4

lf α \displaystyle\alpha and β \displaystyle\beta are the roots of the equation λ ( x 2 + x ) + x + 5 = 0 , \displaystyle\lambda (x^{2} +x) +x+5=0, and λ 1 \displaystyle\lambda_{1} and λ 2 \displaystyle\lambda_{2} are two values of λ \displaystyle\lambda for which α \displaystyle\alpha , β \displaystyle\beta are connected by the relation α β + β α = 4 , \displaystyle\frac{\alpha}{\beta}+\frac{\beta}{\alpha} =4 , then what is the value of λ 1 λ 2 + λ 2 λ 1 ? \displaystyle\frac{\lambda_{1}}{\lambda_{2}}+\frac{\lambda_{2}}{\lambda_{1}} ?


The answer is 782.

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1 solution

Mehul Chaturvedi
Jan 10, 2015

Upvote if you like it


I will try my best to provide a nice and elegant solution

First of all for clarity let λ = a \lambda=a

then we will get a ( x 2 + x ) + x + 5 = 0 a x 2 + x ( a + 1 ) + 5 = 0 a(x^2+x)+x+5=0 \\ ax^2+x(a+1)+5=0

Now by Vieta we have α + β = ( a + 1 ) a α β = 5 a \alpha+\beta=\dfrac{-(a+1)}{a} \\ \alpha\beta=\dfrac{5}{a}

According to question α β + β α = 4 α 2 + β 2 α β = 4 \dfrac{\alpha}{\beta}+\dfrac{\beta}{\alpha}=4 \\ \dfrac{\alpha^2+\beta^2}{\alpha\beta}=4

Now ( α + β ) 2 2 α β α β = 4 \dfrac{(\alpha+\beta)^2-2\alpha\beta}{\alpha\beta}=4

( α + β ) 2 = ( a 2 + 1 + 2 a a 2 ) a 2 + 1 + 2 a 10 a 5 a = 4 ( \alpha+\beta)^2=(\dfrac{a^2+1+2a}{a^2}) \\ \therefore \dfrac{a^2+1+2a-\dfrac{10}{a}}{\dfrac{5}{a}}=4

( a 2 + 1 + 2 a a 2 10 a ) 5 a = 4 ( a 2 + 1 8 a a 2 ) 5 a = 4 a 2 + 1 8 a a 2 × a 5 = 4 a 2 + 1 8 a 5 a = 4 a 2 28 a + 1 = 0 \Rightarrow \dfrac { \left( \dfrac { a^{ 2 }+1+2a }{ a^{ 2 } } -\dfrac { 10 }{ a } \right) }{ \dfrac { 5 }{ a } } =4\\ \Rightarrow \dfrac { \left( \dfrac { a^{ 2 }+1-8a }{ a^{ 2 } } \right) }{ \dfrac { 5 }{ a } } =4\\ \Rightarrow \dfrac { a^{ 2 }+1-8a }{ a^{ 2 } } \times \dfrac { a }{ 5 } =4\\ \Rightarrow \dfrac { a^{ 2 }+1-8a }{ 5a } =4\\ \Rightarrow a^{ 2 }-28a+1=0

Now let roots of this eqn. be ζ \zeta and η \eta (here λ 1 = ζ , λ 2 = η \lambda_1=\zeta,\lambda_2=\eta )

By vieta we have

ζ + η = 28 ζ η = 1 ( ζ + η ) 2 = 784 ζ 2 + η 2 = 784 2 × 1 2 ζ 2 + η 2 = 782 ζ 2 + η 2 ζ η = 782 1 \Rightarrow \zeta +\eta =28\\ \Rightarrow \zeta \eta =1\\ \therefore (\zeta +\eta )^{ 2 }=784\\ \Rightarrow \zeta ^{ 2 }+\eta ^{ 2 }=784-2\times 1^{ 2 }\\ \Rightarrow \zeta ^{ 2 }+\eta ^{ 2 }=782\\ \therefore \dfrac { \zeta ^{ 2 }+\eta ^{ 2 } }{ \zeta \eta } =\dfrac { 782 }{ 1 }

\therefore the answer is 782 \huge\Rightarrow\boxed{\color{royalblue}{782}}


Upvote if you like it

Nicely done ! Upvoted ! :D

Keshav Tiwari - 6 years, 5 months ago

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Thanks for appreciation

Mehul Chaturvedi - 6 years, 5 months ago

Typo here: It should be ( α + β ) 2 2 α β α β = 4 \dfrac{(\alpha+\beta)^2-2\alpha\beta}{\alpha\beta}=4

Other wise same solution , Upvoted

Sualeh Asif - 6 years, 5 months ago

Since I've been using Newton's Identities way too much recently, I used it here too while finding the power sums that you found using a 2 + b 2 = ( a + b ) 2 2 a b a^2+b^2=(a+b)^2-2ab . Other than that, same method as mine. :D

Prasun Biswas - 6 years, 5 months ago

Well explained!!

Aareyan Manzoor - 6 years, 4 months ago

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