Summing the integers.

Algebra Level 4

F i n d t h e s u m o f a l l i n t e g r a l r o o t s o f ( log 5 x ) 2 + log 5 x 5 x = 1 Find\quad the\quad sum\quad of\quad all\quad integral\quad roots\quad of\quad \\ { \left( \log _{ 5 }{ x } \right) }^{ 2 }+\log _{ 5x }{ \frac { 5 }{ x } } =1

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The answer is 6.

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2 solutions

First look at y = log 5 x 5 x ( 5 x ) y = 5 x y = \log_{5x}\dfrac{5}{x} \Longrightarrow (5x)^{y} = \dfrac{5}{x}

x y + 1 = 5 1 y \Longrightarrow x^{y+1} = 5^{1 - y}

( y + 1 ) log 5 ( x ) = 1 y \Longrightarrow (y + 1)\log_{5}(x) = 1 - y

y ( 1 + log 5 ( x ) ) = 1 log 5 ( x ) \Longrightarrow y(1 + \log_{5}(x)) = 1 - \log_{5}(x)

y = 1 log 5 ( x ) 1 + log 5 ( x ) . \Longrightarrow y = \dfrac{1 - \log_{5}(x)}{1 + \log_{5}(x)}.

Now let a = log 5 ( x ) . a = \log_{5}(x). Then the given equation becomes

a 2 + 1 a 1 + a = 1 a 3 + a 2 a + 1 = 1 + a a^{2} + \dfrac{1 - a}{1 + a} = 1 \Longrightarrow a^{3} + a^{2} - a + 1 = 1 + a

a ( a 2 + a 2 ) = 0 a ( a + 2 ) ( a 1 ) = 0. \Longrightarrow a(a^{2} + a - 2) = 0 \Longrightarrow a(a + 2)(a - 1) = 0.

So we have solutions a = log 5 ( x ) = 0 , 1 , 2 x = 1 , 5 , 1 25 . a = \log_{5}(x) = 0, 1, -2 \Longrightarrow x = 1, 5, \dfrac{1}{25}.

The only integral values are 1 1 and 5 5 , the sum of which is 6 . \boxed{6}.

101% overrated!!!utkarsh Bansal

Parth Lohomi - 6 years, 3 months ago

A quality solution, you are great sir.

Shubhendra Singh - 6 years, 3 months ago

nice solution sir. thank you

Utkarsh Bansal - 6 years, 3 months ago
Rohit Ner
Feb 25, 2015

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