When you (like I) think that the trig functions are best understood as segments within the Complete Triangle, you tend to believe that just about every important fact about those functions has a nifty geometric interpretation. (See other posts in this category.) Why shouldn’t we expect this to be true of the functions’ power series? […]

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## The Geometry of the Power Series for Trig Functions

## Proof with Too Many Words: Derivatives of Sine and Cosine

I’d like to think that the following constitutes a Proof without Words of the formulas for the derivatives of sine and cosine. Unfortunately, it seems that words may be in order, so I wrote some here: “Calculus-free Derivatives of Sine and Cosine” … even though, strictly speaking, the proof isn’t entirely calculus-free. (Why we expect the […]

## Proof without Words: The Law of Cosines

(I really need to put this onto a poster.)