Roots

Algebra Level pending

The equation x 2 6 x 2 = 0 x^2 - 6x - 2 = 0 has two roots, α \alpha and β \beta . Let a n = α n + β n a_n = \alpha^n + \beta^n

Find a 100 2 a 98 a 99 \dfrac{a_{100} -2a_{98}}{a_{99}}

This problem is not original.


The answer is 6.

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2 solutions

Pi Han Goh
May 18, 2014

Newton's Sum: S 100 6 S 99 2 S 98 = 0 S 100 2 S 98 S 99 = 6 \large S_{100} - 6S_{99} - 2 S_{98} = 0 \Rightarrow \frac {S_{100}-2S_{98}}{S_{99}} = 6

Same method. :D

Finn Hulse - 7 years ago

its a 2011 IITJEE question

Nikhil PS - 6 years, 10 months ago
Nikhil Ps
Aug 6, 2014

{ x }^{ 2 }-6x-2\quad =\quad 0\ \Longrightarrow { \alpha }^{ 2 }-2\quad =\quad 6\alpha \quad and\quad { \beta }^{ 2 }-2\quad =\quad 6\beta \ \Longrightarrow { \alpha }^{ 100 }+{ \beta }^{ 100 }\quad -\quad 2 { \alpha }^{ 98 }-2 { \beta }^{ 98 }\quad =\quad { \alpha }^{ 98 }({ \alpha }^{ 2 }-2)+{ \beta }^{ 98 }(\beta ^{ 2 }-2)\quad =\quad 6({ \alpha }^{ 99 }+{ \beta }^{ 99 })\ therefore\quad \xleftarrow [ { \alpha }^{ 100 }+{ \beta }^{ 100 }\quad -\quad 2 { \alpha }^{ 98 }-2 { \beta }^{ 98 } ]{ { \alpha }^{ 99 }+{ \beta }^{ 99 } } =\quad \xleftarrow [ 6({ \alpha }^{ 99 }+{ \beta }^{ 99 }) ]{ { \alpha }^{ 99 }+{ \beta }^{ 99 } } =\quad 6

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