Home Light Version IE Version Search | BBS | Guest Book | Site Map | Help | Contact us | About us

### [Index]

Introduction
Essence
Memorizing

Basic Formulas
Sum and Difference Formulas
Double & Triple Angle Formulas
Half Angle Formulas
Product to Sum Formulas
Sum to Product Formulas
All Formulas

Understanding
Summarizing
Examples
Exercises
Final thoughts

## Sum and Difference Formulas:

There are four possible products of two factors that can be formed from , , , and . Namely: , , , . Note their distribution in the first four formulas. The right hand sides of (1) and (2) consist of the algebraic sum of and , while (3) and (4) consist of the mixed products. Note the distribution of signs below.

In formulas 1 and 2, for cosine the signs on each side of the formulas are "opposite". Be aware of the order of the subtraction: the product of cosine minus the product of sine.

In formulas 3 and 4, for sine the signs on both sides are the same. Be aware of the subtraction in formula 4: in front of the product is negative, when the product contains the factor , where is from the term of .

Formulas 5 and 6, tangents have the same sign in the numerator of both sides. The numerator is also what you get if you multiply tangent by (i.e. . Notice, this is not actually how it works). The denominator is , the sign being the opposite of the numerator.