I can understand Anon's frustration back in Jan '16. So often in mathematical explanations I've read I find myself tripping over a missing step. Like a mathematical pothole. It's usually something so obvious to the mathematician who wrote it that it didn't seem to need mentioning. ( Like where that little square came from- though I did eventually work that one out). The problem is that if you are trying to follow a set of mathematical steps even if you solve the missing one (as with me and the small square) you have been diverted away from the main problem and lost the thread: And then probably give up and go off and do something else instead.

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