1. The problem statement, all variables and given/known data I'm reffering to http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dipole.html and the approximation [tex] r_1 - r_2 \approx = d \cos \theta[/tex]. I see that it is correct if I draw it up, but I wondered if there were any "more mathematical" ways to see this? Where does these kind of approximations come from? Is this just a special case or is it often used? I don't think I've seen this kind before.