InfoPath Dev

How can I apply a square root in a formula in InfoPath form? I am trying to calculate the BSA as follows:BSA (m2) = SQRT( [Height(cm) x Weight(kg) ]/ 3600 )

There isn't any out of the box way to do this. But you may be able to leverage Excel services if this is not a browser Office 365 form & you have Excel services available on your SharePoint server.

How about using Visual Basic in InfoPath for square root functions?

Sure, you can use code for this. I'd suggest unless you are a VB.NET programmer that you use C# instead, since you will find more samples in C#. If your form is browser, Office 365, you won't be able to use code.