Nice job of getting .807, but you forgot the - sign. You will then get -22.8 You left off the - sign on the desmos value.

Actually, a more precise value (so you can plug in and check) is -22.8394296944176

The solution would actually be =13/PI()*SIN^{-1}(-13/18)-19.5 (or you can write -13/pi()*SIN^{-1}(13/18)-19.5; I prefer the former; Excel uses asin for arcsin)

As we see sin(2 pi t/26), the period is 26. 26 + -22.8394296944176 = 3.1605703055824

I thus match the other desmos value.

Jonathan Z.

Thanks a lot, you saved my skin!05/16/20