In the picture below, the left side equation is the one my teacher wrote for sample SD, and the right side equation is the one I found online. Which is correct?

[![enter image description here](https://i.stack.imgur.com/u3pjA.png)](https://i.stack.imgur.com/u3pjA.png)

<https://gyazo.com/0e17145acb652c579dd7e809eaa812cb>