Tuesday, September 13, 2011

Solve ln(3x+8)=ln(2x+2)+ln(x-2) for x.

We'll impose the constraints of existence of
logarithms:


3x+8>0


x>-8/3


2x+2>0


x>-1


x-2>0


x>2


The
interval of admissible values for x is (2, +infinite).


Now,
we'll solve the
equation:


ln(3x+8)=ln(2x+2)+ln(x-2)


Since
the logarithms have the matching bases, we'll apply the product rule to the right
side:


ln(3x+8) =
ln[(2x+2)*(x-2)]


Since the bases are matching, we'll apply
one to one rule:


3x + 8 =
[(2x+2)*(x-2)]


We'll remove the brackets from the right
side:


3x + 8 = 2x^2 - 4x + 2x -
4


We'll move all terms to the right side and we'll use
symmetric property:


2x^2 - 4x + 2x - 4 - 3x - 8 =
0


2x^2 - 5x - 12 = 0


We'll
apply quadratic formula;


x1 = [5 + sqrt(25 +
96)]/4


x1 = (5+11)/4


x1 =
16/4


x1 = 4


x2 =
-6/4


x2 =
-3/2


Since the second solution is not in the
interval of admissible values, we'll reject it. The only valid solution is x =
4.

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