This isn't really a 'problem', I'm just trying to follow Feynman's reasoning in section 18-2 of Volume 1 The Feynman Lectures on Physics. I've attached a png of the paragraph in question. I have 2 issues with this: 1. If the length OQ = OP, how can there be a right angle at P(x,y)? 2. I don't understand why QP = r [itex]\Delta\theta[/itex], rather than r * tan ([itex]\Delta\theta[/itex]) Am I missing something? I've checked the latest errata and this isn't mentioned. Thanks for your help.