Saturday, September 6, 2014

Given the basis x and x+2 of logarithms find all real solutions of equation 5 - 2[log(x+2) x + logx (x+2)] = 0

For the beginning, we'll re-arrange the terms of
the equation, by shifting 5 to the right side.


-2[log(x+2)
x + logx (x+2)] = -5


We'll divide by -2 both
sides:


[log(x+2) x + logx (x+2)] =
5/2


Now, we'll impose the constraints of existence of
logarithms:


x>0


x+2>0


x>-2


x
different from 1


x + 2 different from
1


x different from -1.


The
interval of admissible values for x is (0 ;
+infinite)-{1}.


Now, we'll solve the equation. We'll start
by changing the base with by the argument, in the 1st
term:


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


We'll substitute logx (x+2) by the
variable t.


We'll write the equation in
t:


1/t + t = 5/2


We'll
multiply by 2t both sides:


2 + 2t^2 =
5t


We'll move all terms to one
side:


2t^2 - 5t + 2 = 0


We'll
solve the equation applying quadratic formula:


t1 =
[5+sqrt(25 - 16)]/4


t1 =
(5+3)/4


t1 = 2


t2 =
(5-3)/4


t2 = 1/2


But logx
(x+2) = t1 => logx (x+2) = 2


We'll take
antilogarithms:


x + 2 =
x^2


We'll subtract x+2 both
sides:


x^2 - x - 2 = 0


We'll
apply again quadratic formula, to determine x1 and x2:


x1 =
[1+sqrt(1+8)]/2


x1 =
(1+3)/2


x1 = 2


x2 =
(1-3)/2


x2 = -1


We'll reject
the second value of x since it doesn't respect the constraints of existence of
logarithms.


Now, we'll put


But
logx (x+2) = t2 => logx (x+2) = 1/2


We'll take
antilogarithms:


x + 2 =
sqrtx


We'll raise to square both
sides:


(x+2)^2 = x


We'll
expand the square:


x^2 + 4x + 4 =
x


x^2 + 3x + 4 = 0


We'll apply
quadratic formula:


x1 =
[-3+sqrt(9-16)]/2


Since sqrt-7 is undefined, the equation
x^2 + 3x + 4 = 0 has no real solutions.


The
only real solution of the equation is: x = 2.

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