Powers of 2014 , 2015 and 2016

Algebra Level 4

If x 2 3 x + 1 = 0 x ^ 2 - 3x + 1 = 0 has roots α , β \alpha, \beta , then find the value of the expression below?

α 2014 + β 2014 + α 2016 + β 2016 α 2015 + β 2015 \frac{ \alpha^ {2014} + \beta ^ { 2014 } + \alpha^{2016 } + \beta ^ { 2016 } } { \alpha ^ { 2015 } + \beta ^ { 2015 } }


The answer is 3.

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8 solutions

Sandeep Rathod
Feb 25, 2015

α 2 3 α + 1 = 0 \alpha^2 - 3\alpha + 1 =0

Multiplying by α 2014 \alpha^{2014}

α 2016 3 α 2015 + α 2014 = 0 \alpha^{2016} - 3\alpha^{2015} + \alpha^{2014} = 0

Similarly

β 2016 3 β 2015 + β 2014 = 0 \beta^{2016} - 3\beta^{2015} + \beta^{2014} = 0

Adding,

α 2014 + β 2014 + α 2016 + β 2016 = 3 ( α 2015 + β 2015 ) \alpha^{2014} + \beta^{2014} + \alpha^{2016} + \beta^{2016} = 3( \alpha^{2015} + \beta^{2015})

α 2014 + β 2014 + α 2016 + β 2016 α 2015 + β 2015 = 3 \dfrac{\alpha^{2014} + \beta^{2014} + \alpha^{2016} + \beta^{2016} }{\alpha^{2015} + \beta^{2015}} = 3

Yeah! That's the most elegant.

Nihar Mahajan - 6 years, 3 months ago

It should also be specified in the solution that ( α 2015 + β 2015 ) 0 (\alpha^{2015}+\beta^{2015})\neq 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.

Prasun Biswas - 6 years, 3 months ago

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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

sandeep Rathod - 6 years, 3 months ago

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Yes, I know that. I'm saying that you should include this reasoning in your solution.

Prasun Biswas - 6 years, 3 months ago

Beautiful !

Maninder Dhanauta - 6 years, 3 months ago

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. :)

Mary Joyce Italia - 6 years, 3 months ago

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Here a property a m × a n = a m + n a^{m} \times a^{n}=a^{m+n} is used.

Shubhendra Singh - 6 years, 3 months ago
Shubhendra Singh
Feb 25, 2015

The expreesion to be found out cna be written as

α 2014 ( α 2 + 1 ) + β 2014 ( β 2 + 1 ) α 2015 + β 2015 \dfrac{\alpha^{2014}(\alpha^{2}+1)+\beta^{2014}(\beta^{2}+1)}{\alpha^{2015}+\beta^{2015}}

Since α & β \alpha \ \& \ \beta are the roots of x 2 3 x + 1 = 0 x^{2}-3x+1=0

So α 2 3 α + 1 = 0 \alpha^{2}-3\alpha+1=0 and β 2 3 β + 1 = 0 \beta^{2}-3\beta+1=0

By this α 2 + 1 = 3 α \alpha^{2}+1=3\alpha and β 2 + 1 = 3 β \beta^{2}+1=3\beta

Substitute these values in the topmost expression to get

α 2014 ( 3 α ) + β 2014 ( 3 β ) α 2015 + β 2015 \dfrac{\alpha^{2014}(3\alpha)+\beta^{2014}(3\beta)}{\alpha^{2015}+\beta^{2015}}

3 ( α 2015 + β 2015 ) α 2015 + β 2015 \Rightarrow \dfrac{ 3(\alpha^{2015}+\beta^{2015})}{\alpha^{2015}+\beta^{2015}}

So the asnwer is 3 \huge 3

Isha Amoncar
Mar 6, 2015

i did the same way :D

Prachi Garg - 6 years, 3 months ago

Yup, that's what I did

Raushan Sharma - 5 years, 9 months ago
Aareyan Manzoor
Feb 25, 2015

the roots of the polynomial are φ 2 , φ 2 \varphi^2,\varphi^{-2} by identites of the golden ratios, φ n + ( φ ) n = L n \varphi^n+(-\varphi)^{-n}=L_{n} such that L n = { 2 , n = 0 1 , n = 1 L n 1 + L n 2 , n 2 L_{n} =\begin{cases} 2, n=0\\ 1,n=1\\ L_{n-1}+L_{n-2},n\geq 2 \end{cases} hence, φ 4028 + φ 4028 + φ 4032 + φ 4032 φ 4030 + φ 4030 = L 4028 + L 4032 L 4030 \dfrac{\varphi^{4028}+\varphi^{-4028}+\varphi^{4032}+\varphi^{-4032}}{\varphi^{4030}+\varphi^{-4030}}=\dfrac{L_{4028}+L_{4032}}{L_{4030}} rewrite L 4028 + L 4032 L 4030 = ( L 4030 L 4029 ) + ( L 4030 + L 4031 ) L 4030 = 2 L 4030 + L 4031 L 4029 L 4030 = 3 L 4030 L 4030 = 3 \dfrac{L_{4028}+L_{4032}}{L_{4030}}=\dfrac{(L_{4030}-L_{4029})+(L_{4030}+L_{4031})}{L_{4030}}=\dfrac{2L_{4030}+L_{4031}-L_{4029}}{L_{4030}}=\dfrac{3L_{4030}}{L_{4030}}=\boxed{3}

Unnecessary Complication !

Venkata Karthik Bandaru - 6 years, 3 months ago
Gaetano Tarantino
Feb 26, 2016

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.

Saransh Gupya
Mar 9, 2015

Alec Zhang
Feb 27, 2015

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.

Saurav Pal
Feb 26, 2015

α 2014 + β 2014 + α 2016 + β 2016 α 2015 + β 2015 = α 2015 ( α + 1 α ) + β 2015 ( β + 1 β ) α 2015 + β 2015 . 3 ( α 2015 + β 2015 ) ( α 2015 + β 2015 ) = 3. \frac { { \alpha }^{ 2014 }+{ \beta }^{ 2014 }+{ \alpha }^{ 2016 }+{ \beta }^{ 2016 } }{ { \alpha }^{ 2015 }+{ \beta }^{ 2015 } } \quad =\quad \frac { { \alpha }^{ 2015 }(\alpha +\cfrac { 1 }{ \alpha } )+{ \beta }^{ 2015 }(\beta +\cfrac { 1 }{ \beta } ) }{ { \alpha }^{ 2015 }+{ \beta }^{ 2015 } } .\quad \\ \Rightarrow \quad \frac { 3({ \alpha }^{ 2015 }+{ \beta }^{ 2015 }) }{ ({ \alpha }^{ 2015 }+{ \beta }^{ 2015 }) } \quad =\quad 3.\quad \quad

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