Given r=f(θ), we are generally not concerned with r′=f′(θ); that describes how fast r changes with respect to θ
You're using the derivative of a polar equation as the basis for what a tangent line is. But as the textbook explains, that doesn't give you a tangent line or describe the slope at that point. I never bothered defining what "tangent" means, but since this seems so important to you why don't you try coming up with a reasonable definition?
https://www.iso-ne.com/ Looking at my own region of New England, renewables are only at about 8% right now. And that includes burning wood, refuse, and landfill gas as renewable sources.