Algebra
Every time I read such lucid explanations of math like this I'm filled with resentment for the math instruction I received in junior high and high school in the U.S. There's a real sense of 'play' that these explanations evoke and that make thinking about numbers and their relationships genuinely fun and interesting. That sense of play was entirely absent from my early math education. It was all "These are the rules. Memorize them for the quiz tomorrow."I can't help feeling like my math upbringing was akin to a child being raised by parents who speak their own made-up language. Integrating with the rest of the normal-language-speaking world is anxiety inducing and filled with challenges that may never be completely overcome.