I tutor math and science and they struggle with keeping all of the pre-cal formulas straight. This one page isn't anything that students couldn't create through the year as they are used; there are also commercial versions with the same information, but this is what I give my tutoring students and they greatly appreciate it. Plus if they lose it, I just print them another one. It is also a great reference at the end of the semesters when they are doing their final reviews.
I usually put a copy of the unit circle on the back (although there are plenty of pdf files online for that, so why pay me when you can get it easily online!)
I have included all the formulas we used through the year - including:
Area of a triangle (using trig functions)
Law of Cosines
Change of base formula (logs)
Pythagorean Theorem (using trig functions)
Converting Polar to Rectangular coordinates
Operations with functions
Sin/cos curve transformations
Double Angle Formula
This is a word document so you could tweak and add any that might be missing but important to your classes!
NOTE: I support copyright and trademark infringement free products. To that end, I have confirmed, all fonts, photos, clip art used in the creation of this product and its cover page (if applicable) are all public domain, royalty-free. So purchase with the assurance that this product is my own work and all graphics are copyright/trademark free.