I wonder how the LHS equation changes to the RHS, because I think there should be no "n" in the denominator on the LHS, so why on the RHS there will be a variance in the numerator?

[![enter image description here](https://i.stack.imgur.com/fZbwM.png)](https://i.stack.imgur.com/fZbwM.png) -> [![enter image description here](https://i.stack.imgur.com/NWb6d.png)](https://i.stack.imgur.com/NWb6d.png)