A new be question - Live update of a numeric form field

A new be question - Live update of a numeric form field

I would like to be able calculate cost for items and see the tax amount in a field and the total in a field, but I'm not sure hoe to do this, with out using the function field, the going to the view.

cost - $100.00 --- I enter
tax   -      8.25 --- auto calculate
total - $108.25 --- auto calculate 

thanks