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

   Final thoughts

Half-Angle Formulas:

3_301.gif (11236 bytes)

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 3_302.gif (1247 bytes), 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 alpha_2.gif (954 bytes) and which quadrant alpha_2.gif (954 bytes) is in. For example, if alpha_2.gif (954 bytes) is in second quadrant, then sin_al_2.gif (1171 bytes) has positive sign and cos_al_2.gif (1186 bytes), tan_al_2.gif (1204 bytes) 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.


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