I was reading a book and came across with a equation which gives the normal distribution function of continuous random variable. It was used in a software called `RapidMiner`

to visualize data distribution.

$f\left( x \right) = \frac{1}{{\sqrt {2\pi \sigma } }}{e^{\frac{{{{\left( {x - \mu } \right)}^2}}}{{2{\sigma ^2}}}}}$ , where $x$ is random variable, $\sigma$ is the standard deviation, and $\mu$ is the mean of the distribution.

I got badly confused, as it is different and not same as what I learnt from textbook or wikipedia, which is:

$f\left( x \right) = \frac{1}{{\sigma \sqrt {2\pi } }}{e^{ - \frac{{{{\left( {x - \mu } \right)}^2}}}{{2{\sigma ^2}}}}}$ where the symbols mean the same.

**I have questions:**

- is the equation in the book correct?
- if so, could you please walk me through how could the equation be correct? Maybe give me some keyword for searching.
- if wrong, could you tell me why?

Finally the book luckily provide a on-line reading site: http://www.learnpredictiveanalytics.com/preview.html . The context of the equation of my question is on Page 51, right below figure 3.9.

Thank you.