Take a look at FNC-Elcad. It will design a circuit for you and then
display all the formulas used. For example, if you want to design a
power supply, just enter in your requirements and it will display the
schematic with all the right component values. Click on "Formulas"
and it will display all the formulas used to design the power supply.
While it is not free, it is very low priced. It has a free trial version,
you can download and try.
Take a look at it at http://www.fncwired.com
Hope this helps