Well, it’s so much easier and more robust that way! Instead of a long list of confoundingly similar equations, you’re left with a single clear understanding of why trigonometry works. After that you can memorize a few formulas as shortcuts if it helps.
Of course this principle completely breaks down when you start working with a child who’s already convinced that they can’t do math—or with a group of 30 kids at once, a third of whose mathematical intuitions will be far enough from the textbook norm that no one teacher has enough time to guide them through to that first epiphany.
Well, it’s so much easier and more robust that way! Instead of a long list of confoundingly similar equations, you’re left with a single clear understanding of why trigonometry works. After that you can memorize a few formulas as shortcuts if it helps.
Of course this principle completely breaks down when you start working with a child who’s already convinced that they can’t do math—or with a group of 30 kids at once, a third of whose mathematical intuitions will be far enough from the textbook norm that no one teacher has enough time to guide them through to that first epiphany.