# Thread: help with lo equation

1. ## help with lo equation

hi, after 30 years from leaving school, ive decided to to a night class in engeineering and weve just started a module on maths. but im finding the maths very difficult. one of the questions is simplyfy 36 x 42 / 56 . using the log laws. ive never done logs before and im trying to read about them but i keep getting confused. Any help would be appreciated.

2. The main formulas you need here are the following:
$\displaystyle ln(ab) = ln(a)+ln(b)$
$\displaystyle ln(\frac{a}{b}) = ln(a)-ln(b)$
$\displaystyle ln(a^c) = cln(a)$.
In the example you gave, take the logarithm: you get the following:
$\displaystyle ln(\frac{36\times 42}{56}) = ln(36)+ln(42)-ln(56)$
Now decompose each number using its prime factorization:
you get: $\displaystyle ln(2^2\times 3^2)+ln(2\times 3\times 7)-ln(2^3 \times 7)$ $\displaystyle = 2ln(2)+2ln(3)+ln(2)+ln(3)+ln(7)-3ln(2)-ln(7) = 3ln(3)=ln(3^3) = ln(27).$
Hence, the number you need is 27.

Hope this helps

3. log(27) = 3.2958....

log(36) * log(42) / log(56) = 3.3274....

?

4. $\displaystyle ln(\frac{36\times 42}{56}) = ln(36)+ln(42)-ln(56)$ and not $\displaystyle \frac{ln(36)*ln(42)}{ln(56)}$

5. Agree if looked at that way; I guess it depends HOW one reads the problem...

Thanks Mr Fawaz.

The original post: simplyfy 36 x 42 / 56 . using the log laws.
You don't need log laws to simplify 36 * 42 / 56: simple calculation...
By the way, 36 * 42 / 56 = 27 : comments?

6. Hehehehe, I was sure that what you posted was kind of for fun and that you were not actually wondering how the results are different, and that is based on the number of posts and thanks you have. I replied to your post to avoid confusion since lukebushnell has never done logs before.

Hehehehe, I was sure that what you posted was kind of for fun...
Yes Mohammad, it sort of was: I really was trying to show the poster
that his question was quite UNCLEAR...