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 note logx (x+2) =
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
apply 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 quadratic formula:
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
equation will have only one valid solution: x =
2.
No comments:
Post a Comment