Can you include X1/X2 in a model when you've already included X1 and X2? For example, I am trying to predict housing prices. X1=square feet and X2=lot size. I want to see how much of the lot isn't being used as a proxy for how big the front and back yard are. Thus, I want to use X1/X2 since this is my only data. Can I include X1/X2 when I have already included X1 and X2?