Physics anyone? LOL

In your equation:
k= Rd/(T2-T1)A

k = Thermal Conductivity.
I can only assume that the Rd variable is some sort of thermal resistance property. T1 and T2 being the temperatures.

It's been many years since I've dealt with this stuff, but it illustrates the main peeve I've always had with people teaching using equations. Students have no idea what the equation means, and instead of understanding the concepts they just have a list of equations and "when to use them". When I took my physics courses in high school and college I had classmates that had pages filled with equations, many of them were the same equation just wrote out differently. It annoyed me to no end.

With a little understanding, some algebra, and a few basic equations, you could easily get everything those pages of equations contained. There were times when I forgot the equations for certain things, and were able to piece them together using nothing but the units of the answer. That's something important too, understanding that your units must match up at the end as well.