For Precalc H, I created a series of dynamic Geogebra files, where the teacher or student uses sliders to control the center and the a (and b) values of the conic. The file calculates and graphs the appropriate conic, foci, directrices, etc.
All the files can be downloaded here.
Sample:
Or you can go to geogebratube to run them in your web browser.
Cheers.