Math Medics
S.o.s. Math: Hyperbolic Functions
This math lesson gives formulas for the hyperbolic trigonometric functions, a very most important property of their version of the Pythagorean Theorem, and it's verification, and many helpful visual aides.
McGraw Hill
Mc Graw Hill: The Additive Inverse Property
The sum of any number and its additive inverse is always a zero. This site gives you the rule for this property.