If x 2 − 3 x + 1 = 0 has roots α , β , then find the value of the expression below?
α 2 0 1 5 + β 2 0 1 5 α 2 0 1 4 + β 2 0 1 4 + α 2 0 1 6 + β 2 0 1 6
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Yeah! That's the most elegant.
It should also be specified in the solution that ( α 2 0 1 5 + β 2 0 1 5 ) = 0 so as to justify the cancellation of the term from numerator and denominator and also that the given expression is defined.
Showing that the sum of the roots is non-zero and none of the roots can be zero suffices to prove the claim.
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Now sum of the roots is positive and product is positive too , so both are positive , thus theirs power to 2015 will never be equal to zero
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Yes, I know that. I'm saying that you should include this reasoning in your solution.
Beautiful !
Hey. I'm just confuse. According to the solution, "Multiplying by a^2014". How did it become a^2016-3a^2015+a^2014. Need response. Thanks. :)
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Here a property a m × a n = a m + n is used.
The expreesion to be found out cna be written as
α 2 0 1 5 + β 2 0 1 5 α 2 0 1 4 ( α 2 + 1 ) + β 2 0 1 4 ( β 2 + 1 )
Since α & β are the roots of x 2 − 3 x + 1 = 0
So α 2 − 3 α + 1 = 0 and β 2 − 3 β + 1 = 0
By this α 2 + 1 = 3 α and β 2 + 1 = 3 β
Substitute these values in the topmost expression to get
α 2 0 1 5 + β 2 0 1 5 α 2 0 1 4 ( 3 α ) + β 2 0 1 4 ( 3 β )
⇒ α 2 0 1 5 + β 2 0 1 5 3 ( α 2 0 1 5 + β 2 0 1 5 )
So the asnwer is 3
i did the same way :D
Yup, that's what I did
the roots of the polynomial are φ 2 , φ − 2 by identites of the golden ratios, φ n + ( − φ ) − n = L n such that L n = ⎩ ⎪ ⎨ ⎪ ⎧ 2 , n = 0 1 , n = 1 L n − 1 + L n − 2 , n ≥ 2 hence, φ 4 0 3 0 + φ − 4 0 3 0 φ 4 0 2 8 + φ − 4 0 2 8 + φ 4 0 3 2 + φ − 4 0 3 2 = L 4 0 3 0 L 4 0 2 8 + L 4 0 3 2 rewrite L 4 0 3 0 L 4 0 2 8 + L 4 0 3 2 = L 4 0 3 0 ( L 4 0 3 0 − L 4 0 2 9 ) + ( L 4 0 3 0 + L 4 0 3 1 ) = L 4 0 3 0 2 L 4 0 3 0 + L 4 0 3 1 − L 4 0 2 9 = L 4 0 3 0 3 L 4 0 3 0 = 3
Unnecessary Complication !
I just thought: alpha is between 0 and 1, beta is between 2 and 3, the numerator and the denominator are mainly defined by giant powers of beta so at least the answer is beta, but it's a tiny bit more because of a lower power in sum above, and it couldn't be more than beta squared; so the plausible rough answers were 3, 4, and 5; 5 seemed too close to beta squared, four seemed too much, 3 made sense.
Using the Newton's Sums Method, we see that for n >= 3, S(n) = 3S(n-1)-S(n-2), where S(n) = a^n + b^n. Thus, S(2016) = 3S(2015)-S(2014), and the fraction becomes (S(2014)+S(2016))/S(2015) = 3S(2015)/S(2015) = 3.
α 2 0 1 5 + β 2 0 1 5 α 2 0 1 4 + β 2 0 1 4 + α 2 0 1 6 + β 2 0 1 6 = α 2 0 1 5 + β 2 0 1 5 α 2 0 1 5 ( α + α 1 ) + β 2 0 1 5 ( β + β 1 ) . ⇒ ( α 2 0 1 5 + β 2 0 1 5 ) 3 ( α 2 0 1 5 + β 2 0 1 5 ) = 3 .
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α 2 − 3 α + 1 = 0
Multiplying by α 2 0 1 4
α 2 0 1 6 − 3 α 2 0 1 5 + α 2 0 1 4 = 0
Similarly
β 2 0 1 6 − 3 β 2 0 1 5 + β 2 0 1 4 = 0
Adding,
α 2 0 1 4 + β 2 0 1 4 + α 2 0 1 6 + β 2 0 1 6 = 3 ( α 2 0 1 5 + β 2 0 1 5 )
α 2 0 1 5 + β 2 0 1 5 α 2 0 1 4 + β 2 0 1 4 + α 2 0 1 6 + β 2 0 1 6 = 3