What you'll learn

21 lessons in Why

What a ratio actually isWhy you cannot divide by zeroWhy a negative times a negative is positiveScaling, and what similar meansWhy every circle has the same $\pi$Pinning $\pi$ down by handWhy the same $\pi$ shows up in the areaThe radian, and $\pi$ as half a turn$\pi$ is the constant of turningSine and cosine are what a turn doesWhat $dy/dx$ actually meansThe equation that makes $\pi$ appearWhy $e^{i\pi} = -1$$\pi$ in a sum of fractions$\pi$ under the bell curve$\pi$ in the normal distribution$\pi$ from dropping a needleThe rule behind all of itThe case for $\tau = 2\pi$What $\pi$ being transcendental costs$\pi$ to seven places, by hand, in 1706

How Erudia teaches

Built to be understood — and remembered.

Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Why exactly where it's useful.

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Related Foundations subjects

Reading Mathematics Thinking Mathematical Logic Set theory & foundations Model Theory Computability Theory