Sparked by a conversation this past weekend about the usefulness of the half-angle identities, I constructed geometric proofs for Image may be NSFW.
Clik here to view.
and Image may be NSFW.
Clik here to view.
. 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.
Image may be NSFW.
Clik here to view.
________________________________________________________
Image may be NSFW.
Clik here to view.
__________________________________________________________
Image may be NSFW.
Clik here to view.
_____________________________________________________________
Image may be NSFW.
Clik here to view.
________________________________________________________________
Image may be NSFW.
Clik here to view.
_______________________________________________________________Image may be NSFW.
Clik here to view.
______________________________________________________________
Image may be NSFW.
Clik here to view.
_________________________________________________________________
Image may be NSFW.
Clik here to view.
______________________________________________________________
Though this one is my favorite:
Image may be NSFW.
Clik here to view.
____________________________________________________________________________
Image may be NSFW.
Clik here to view.
_______________________________________________________________________________________
Partially because of the way it naturally generalizes into the proof of the derivative of sine. If you just let Image may be NSFW.
Clik here to view.
approach 0, Image may be NSFW.
Clik here to view.
approaches 1 and that point in the interior of the circle ends up on the circle, where Image may be NSFW.
Clik here to view.
merges with Image may be NSFW.
Clik here to view.
itself.
Image may be NSFW.
Clik here to view.
_____________________________________________________________________________________
Image may be NSFW.
Clik here to view.
______________________________________________________________________________________
Image may be NSFW.
Clik here to view.
_______________________________________________________________________________________
Image may be NSFW.
Clik here to view.
___________________________________________________________________________________
Image may be NSFW.
Clik here to view.
__________________________________________________________________________________
Image may be NSFW.
Clik here to view.
___________________________________________________________________________________________
Image may be NSFW.
Clik here to view.
____________________________________________________________________________________
Image may be NSFW.
Clik here to view.
____________________________________________________________________________________
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!
Image may be NSFW.
Clik here to view.
____________________________________________________________________________________
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.
Image may be NSFW.
Clik here to view.
_________________________________________________________
Image may be NSFW.
Clik here to view.
____________________________________________________________________________________
Image may be NSFW.
Clik here to view.
_________________________________________________________________________________________
Image may be NSFW.
Clik here to view.