# Thread: Why is 10^lg 5=5 and e^ln5=5, etc?

1. ## Why is 10^lg 5=5 and e^ln5=5, etc?

Why is

$10^(lg{5})=5$
$e^(ln{5})=5$

so on and so forth?

Thanks.

2. u have a strange name
anyway

$log x = log_{10} x$ when u did not write the base it is 10

$log_a a = 1$ in general

if u do not know what is the logarithm this is an example
suppose $x^a = b$ then $a = \log _x b$

so
to find $10^{\log 5}$

let $x = 10^{\log_{10} 5}$

$\log _{10} x = \log _{10} 5$ so $x = 5$

second one same as the first one

$ln = \log _{e}$

3. lg means base 10 already. -.-

4. Originally Posted by Amer
u have a strange name
anyway

$log x = log_{10} x$ when u did not write the base it is 10

$log_a a = 1$ in general

if u do not know what is the logarithm this is an example
suppose $x^a = b$ then $a = \log _x b$

so
to find $10^{\log 5}$

let $x = 10^{\log_{10} 5}$

$\log _{10} x = \log _{10} 5$ so $x = 5$

second one same as the first one

$ln = \log _{e}$
Your explanation is unclear but u gave me inspiration to understand. THANKS

5. Specifically, $log_a(x)$ or "logarithm to base a of x" is defined as the "inverse function" to $a^x$. That is, each "undoes" the other: $a^{log_a(x)}= x$ and $log_a(a^x)= x$.

Your "lg" is an abbreviation for the "common logarithm" or "logarithm base 10", $log_{10}(x)$ which is the inverse to $10^x$. $lg(10^x)= log_{10}(10^x)= x$ for all x (and so for x= 5 $lg(10^5)= 5$). $10^{lg(x)}= 10^{log_{10}(x)}= x$ for all positive x (and so for x= 5 $5^{lg(5)}= 5$.

"ln" is an abbreviation for the "natural logarithm" or "logarithm base e" (e is about 2.718...) which is inverse to $e^x$. $ln(e^x)= log_e(e^x)= x$ for all x (and so for x= 5 $ln(e^5)= 5$. $e^{ln(x)}= x$ for all positive x (and so for x= 5 $e^{ln(x)}= x$.)

(Note the difference between "for all x" and "for positive x". The function " $a^x$ is only defined for positive a and always has a positive value- that is, $a^x$ is a function from the set of all real numbers to the set of all positive real numbers. It's inverse, $ln_a(x)$, is a function from the set of all positive real numbers to the set of real numbers.)

6. how do you guys learn to write the formula source code? I actually trial and error. Is there anyway to learn?

7. Originally Posted by stupidguy
how do you guys learn to write the formula source code? I actually trial and error. Is there anyway to learn?
It is LaTeX.

8. Originally Posted by Plato
It is LaTeX.
So am I supposed to memorize the list of symbols and characters in order to use? Wouldn't that be too tedious?

9. Originally Posted by stupidguy
So am I supposed to memorize the list of symbols and characters in order to use? Wouldn't that be too tedious?
It may be tedious, but it is well worth it.
Posters who use LaTeX usually get faster and better help.