# Logarithm question [won't take two seconds!]

• March 26th 2009, 09:05 AM
db5vry
Logarithm question [won't take two seconds!]

$log_a (3x + 4) - log_a x = 3 log_a 2$

I have got to

$log_a \frac{3x + 4}{x} = log_a 8$

But I'm not sure if this is the correct method or what to do after this to solve it.

• March 26th 2009, 09:14 AM
running-gag
Hi

I would advise first to find the domain of $log_a (3x + 4) - log_a x = 3 log_a 2$

$3x + 4 > 0$ and $x > 0$ therefore $x > 0$

Then OK for your method which leads to $\frac{3x + 4}{x} = 8$

Solve for x and do not forget to check if $x > 0$
• March 26th 2009, 09:31 AM
db5vry
Quote:

Originally Posted by running-gag
Hi

I would advise first to find the domain of $log_a (3x + 4) - log_a x = 3 log_a 2$

$3x + 4 > 0$ and $x > 0$ therefore $x > 0$

Then OK for your method which leads to $\frac{3x + 4}{x} = 8$

Solve for x and do not forget to check if $x > 0$

I got 0.8 for x because I realized just now the method I needed! Thanks for helping.
• March 26th 2009, 09:40 AM
running-gag
Quote:

Originally Posted by db5vry
I got 0.8 for x because I realized just now the method I needed! Thanks for helping.

That's it (Clapping)