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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