Friday, October 21, 2011

What are the solutions of the equation logx (2)+log2x (2)=log4x (2)?

We'll recall the identity: log a (b) = 1/log b
(a)


Therefore, using this property, we'll create matching
bases in the given equation:


1/ log 2 (x) + 1/log 2 (2x) =
1/log 2 (4x)


We'll recall the product property of the
logarithms, such as:


log 2 (2x) = log 2 (2) + log 2
(x)


log 2 (2x) = 1 + log 2
(x)


log 2 (4x) = log 2 (4) + log 2
(x)


log 2 (4x) = 2 + log 2
(x)


We'll substitute log 2 (x) by
t:


1/t + 1/(1 + t) =
1/(2+t)


The equation will
become:


(1+t)(2+t) + t(2+t) - t(1+t) =
0


We'll remove the brackets:


2
+ 3t + t^2 + 2t + t^2 -t - t^2 = 0


We'll eliminate like
terms:


t^2 + 3t + 2 + 2t - t =
0


We'll combine like
terms:


t^2 + 4t + 2  = 0


We'll
apply quadratic formula:


t1 = [-4 + sqrt(16 -
8)]/2


t1 = (-4 + 2sqrt2)/2


t1
= sqrt2 - 2


t2 = -sqrt2 -
2


log 2 (x) = sqrt2 - 2 => x = 2^(sqrt2 -
2)


log 2 (x) = -(sqrt2 + 2) => x = 1/2^(sqrt2 +
2)


Since the x values are positive and they
are different from 1, we'll accept them as solutions of the given equation: {2^(sqrt2 -
2) ; 1/2^(sqrt2 + 2)}.

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