411 REFERENCE · Wikibooks
Trigonometry/More About Addition Formulas
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
Related Formulae
Tangent Formulae
In the last row of expressions, if then the trigonometric functions are all positive so the positive sign is needed before the square root.
Derivations
Using cofunctions we know that . Use the formula for and cofunctions we can write
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Having derived we replace with and use the fact that cosine is even and sine is odd.
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Related Formulae
Derivations
Using and the fact that cosine is even and sine is odd, we have
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Addition Formula for Cosines Double-Angle Formulas