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### [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

## Half-Angle Formulas:

Formulas 1, 2 and 3 are all fractions of cosine functions. Note the order of 1 and cosine: is behind 1. The sine squared and tangent squared have negative signs while cosine squared has a positive sign in the numerators. Only tangent squared has the denominator of , the rest have a denominator of 2.

Formulas 4, 5 and 6, these are the same as the squared formulas except everything is square-rooted and also don't forget the "+" and "-" signs, which depend on the functions of and which quadrant is in. For example, if is in second quadrant, then has positive sign and , have negative signs. Here is a way to remember, put one of the four words in "All Students Try Cheating" in the order of quadrant I, II, III, IV. And the first letter of each word represents the first letter of I. All positive, II. Sine positive, III. Tangent, IV. Cosine.