# Homework Help: Exponential equation

1. Jul 5, 2010

### DTA

1. The problem statement, all variables and given/known data

Solve for x:

4x+7=18

2. Relevant equations

It becomes 4x=11

3. The attempt at a solution

I have no idea where to go from here. We never had a problem with such an immovable number such as 11 to work with. Calculators are not allowed to be used either.

2. Jul 5, 2010

### Dickfore

Use the definition of a logarithm.

3. Jul 5, 2010

### DTA

Okay, I think I'm on the right track but I'm still not sure where to go with this:

4log411= ?

4. Jul 5, 2010

### Dickfore

For logarithms it is always true that:

$$a^{x} = b \Leftrightarrow x = \log_{a}{b}, \; a, b > 0, \; a \neq 1$$

From here we have the fundmanetal property of logarithms:

$$a^{\log_{a} b} = b$$

5. Jul 5, 2010

### LCKurtz

Take the log of both sides to any base and solve for x. It doesn't have to be base 4 and if you were after a decimal approximation you would likely want to use base e or base 10.

6. Jul 6, 2010

### DTA

Okay, so I attempted a change of base and got this:

ln11/ln4 ~ 1.729

Is this how it eventually would end up?

Thanks for all your help! Logarithms are just driving me nuts for some reason

7. Jul 6, 2010

yes.