You need to remove the DCU file and I would like to see some unit tests.

Uploaded DCU after last bigger Update 😓
Merged latest changes and uploaded the proper file.
Didn't create test cases when developing as I used it for a Calculator supporting regular- and big-types - Just eye-balled the results to be equal for both types.
Also have no clue how DUnit Testing works tbh.

I did a quick test of the sin function vs the Windows calculator.
Sin(1)
0.8414709848078965066525023216303
but the function returns
0.8414709848771047991823601743884
which only has a few digits of accuracy. Running the System.Math function returns
0.8414709848078965066645908132958
with MORE accuracy than your implementation

Looking at your function you are using some constants like 1.27323954473516

Which is probably introducing the inaccuracy.