#math

Compound interest is earning returns on your returns. It feels like magic because humans think in straight lines and compounding curves upward. The one variable that decides everything is time, and it's the one you can't buy back.

The quotient rule is the calculus shortcut for taking the derivative of a fraction where both the top and the bottom depend on x. It's mostly memorization, but understanding why the formula has the form it does makes the rule much harder to forget.