lf α and β are the roots of the equation λ ( x 2 + x ) + x + 5 = 0 , and λ 1 and λ 2 are two values of λ for which α , β are connected by the relation β α + α β = 4 , then what is the value of λ 2 λ 1 + λ 1 λ 2 ?
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Nicely done ! Upvoted ! :D
Typo here: It should be α β ( α + β ) 2 − 2 α β = 4
Other wise same solution , Upvoted
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 . Other than that, same method as mine. :D
Well explained!!
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I will try my best to provide a nice and elegant solution
First of all for clarity let λ = a
then we will get a ( x 2 + x ) + x + 5 = 0 a x 2 + x ( a + 1 ) + 5 = 0
Now by Vieta we have α + β = a − ( a + 1 ) α β = a 5
According to question β α + α β = 4 α β α 2 + β 2 = 4
Now α β ( α + β ) 2 − 2 α β = 4
( α + β ) 2 = ( a 2 a 2 + 1 + 2 a ) ∴ a 5 a 2 + 1 + 2 a − a 1 0 = 4
⇒ a 5 ( a 2 a 2 + 1 + 2 a − a 1 0 ) = 4 ⇒ a 5 ( a 2 a 2 + 1 − 8 a ) = 4 ⇒ a 2 a 2 + 1 − 8 a × 5 a = 4 ⇒ 5 a a 2 + 1 − 8 a = 4 ⇒ a 2 − 2 8 a + 1 = 0
Now let roots of this eqn. be ζ and η (here λ 1 = ζ , λ 2 = η )
By vieta we have
⇒ ζ + η = 2 8 ⇒ ζ η = 1 ∴ ( ζ + η ) 2 = 7 8 4 ⇒ ζ 2 + η 2 = 7 8 4 − 2 × 1 2 ⇒ ζ 2 + η 2 = 7 8 2 ∴ ζ η ζ 2 + η 2 = 1 7 8 2
∴ the answer is ⇒ 7 8 2
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