Sparked by a conversation this past weekend about the usefulness of the half-angle identities, I constructed geometric proofs for and . Since I’ve never seen these anywhere before, I thought I’d share.

And while I was at it, I thought I’d share all my other geometric proofs, so here they are, posted mostly without comment.

Some of these are so well-known as to be not worth mentioning. Many of them have been stolen from Proofs Without Words I or Proofs Without Words II. I came up with a few of them myself. Frustratingly, almost none of them are to be found in Precalculus textbooks, where they might be learned and appreciated.

________________________________________________________

__________________________________________________________

_____________________________________________________________

________________________________________________________________

_______________________________________________________________

______________________________________________________________

_________________________________________________________________

______________________________________________________________

Though this one is my favorite:

____________________________________________________________________________

_______________________________________________________________________________________

Partially because of the way it naturally generalizes into the proof of the derivative of sine. If you just let approach 0, approaches 1 and that point in the interior of the circle ends up on the circle, where merges with itself.

_____________________________________________________________________________________

______________________________________________________________________________________

_______________________________________________________________________________________

___________________________________________________________________________________

__________________________________________________________________________________

___________________________________________________________________________________________

____________________________________________________________________________________

____________________________________________________________________________________

And finally, one that shows that the sum of a sine and cosine function of the same argument is also a sinusoid. Since I lost the original picture and don’t feel like remaking it, you’ll have to complete the proof on your own!

____________________________________________________________________________________

Update: After some feedback on twitter, I’ve decided to add a few more diagrams. Tim Brzezinski sent me a link to his website of geometric proofs of trig identities and he had some that I’ve never seen before.

Check it out!

https://www.geogebra.org/m/DxAcj8E2#material/QedMT7Pw

I’ve taken two of his diagrams and added them below.

_________________________________________________________

____________________________________________________________________________________

_________________________________________________________________________________________

These are beautiful, Will Rose!! 😀

I have some proofs, are they known?

Link: https://geometriadominicana.blogspot.com/2020/06/geometric-proof-of-sum-angle-formula.html