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by LolWolf 3117 days ago
Just for funsies (but may answer your question): every (differentiable) function is linear at a small enough time scale. This includes e^x or pretty much every function you listed.
3 comments

Which is what differential geometry is based on - you take a surface (for example - the one dimensional functions you mentioned work too, but those are too trivial). Then you associate to each point a plane (re-centered at to the origin). The collection of all vectors that fit into the plane is a vector space called a tangent space, and the collection of all tangent spaces is a tangent bundle. And now you've set up differential geometry and can study it.
Only on infinitely small subintervals of x.
This approximation has vanishing error from a linear function (as the interval decreases) and (most importantly!) we only have finite noisy samples; so it’s essentially indistinguishable (in a statistical way) for a small enough interval given some variance.
sin(x) = x for very small x, this is a useful tool for several proofs.
for all functions, f(x) = f(0) + f'(0) * x for x very close to 0
The approximation is particularly good for sin(x) because the next term in the Taylor series, f''(0) * x^2 / 2, happens to be 0. So the error is O(x^3) rather than the more common O(x^2).
This is a specific instance of what LolWolf just said in the grandparent.
yeah I didn't know that part, just the more specific one.
It's the first term of the Maclaurin series

sin(x) = sin(0) + cos(1)x = 0 + x