sin²(x)+cos²(x) = 1 isn't 𝑗𝑢𝑠𝑡 Pythagorean theorem on the unit circle It also tells us that a rotation (eⁱˣ) followed by its inverse rotation (e⁻ⁱˣ) is the same as no rotation at allpic.twitter.com/INSZcTxtXr
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I don’t get your point. All you need is the fact that they are equivalent, which you get from the ODE.
When you compute the derivative of sin and cos (of the geometric functions, I mean) you are using the properties of the circle in order to approximate the arcs, hence (implicitly) Pithagorean theorem.
You need to know that sin(t)/t tends to 1 when t goes to 0. Otherwise you cannot differentiate sin. And you *need* trigonometry in order to know this. And trigonometry IS Pythagorean identity.
Writing down your ODE for the geometric circular functions requires Pythagorean theorem at the very beginning.
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