cosine seems more primitive than sine
It's a minor quibble. I think of cosine as the real part of e^ix, which is a very simple concept in my head. sine is the imaginary part of e^ix divided by i, which is slightly more complicated. If you had to relegate one to co- status, I'd choose sine.
Describing sine and cosine this way, instead of in terms of triangles, suddenly makes their behavior feel much more intuitive to me; on par with the way complex numbers in general suddenly made sense when someone here described multiplication by i as a rotation.
Thanks.
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