# Basic logarithm

1. Sep 10, 2010

### rajatgl16

I'm very new to logarithm.
I've got homework to solve a ques using logarithm i.e multipication of 9356 by 0.396. And i've done it in following manner

9356*0.396
=log 9356 + log 0.396
=3.9711-1.4023
=2.5688

I know i've done it in a wrong manner as the ans is wrong. I'm very new to it. I just know how to see logarithmic table and know basic formulae only. I just want to know the procedure to do such questions

2. Sep 10, 2010

### Staff: Mentor

It should be obvious that you've done something wrong, since you have 9356 * .396 being equal to 2.5688.

What you're really doing, but not showing, is taking the log of 9356 * .396.

There's another error -- log(.396) is not -1.4023.

3. Sep 10, 2010

### rajatgl16

Then what to do sir, this is my first ques of such a kind.
I've to calculate using logarithm. Please tell me the first step, rest of all i think i'll do myself.
Should i've to do log9356*log0.396?

4. Sep 10, 2010

### rajatgl16

Hey this time i got right answer.
I do it in this way
I assume ans to b x then,
Log(9356*0.396)=log x
3.5688=log x
x=antilog 3.5688= 3705
And i thnk this is right

5. Sep 10, 2010

### Staff: Mentor

You should have
log(9356*0.396)
= log 9356 + log 0.396
Now continue from there.

When you're done, you will have a number that is the logarithm of (9356*0.396).

Presumably the problem wants you to find 9356*0.396, which is easy enough to do using multiplication (by hand or using a calculator), but more difficult if you have to use logs to get it.

Let's call your answer L, where L = log(9356*0.396). To find (9356*0.396), exponentiate each side.
L = log(9356*0.396)
<==> 10L = 10log(9356*0.396)

Hopefully, you know enough about the properties of logs to be able to simplify the right side.

6. Sep 11, 2010

### HallsofIvy

Staff Emeritus
Are they still teaching the use of logarithms for problems like that? With calculators as common as they are now, I would think using logarithms for multiplications and divisions would be outmoded.

(I am reminded of a science fiction story, I think by Isaac Asimov, taking place hundreds of years in the future, in which an engineer whips out his slide rule!)

7. Sep 11, 2010

### skeptic2

Not in electronics. Logarithms are still used extensively in electronics as dB. I use them nearly every day.

A good example of doing multiplication and division by adding and subtracting logarithms.