Cosine of Sum/Mistake

Source Work

 * compound angle formulae (in trigonometry)
 * compound angle formulae (in trigonometry)

Mistake

 * $\map \cos {a \pm b} = \cos a \cos b \pm \sin a \sin b$

Correction
This should read:


 * $\map \cos {a \pm b} = \cos a \cos b \mp \sin a \sin b$

Otherwise it would be interpreted as:


 * $\map \cos {a + b} = \cos a \cos b + \sin a \sin b$
 * $\map \cos {a - b} = \cos a \cos b - \sin a \sin b$

which is incorrect.

The correct identities are:


 * $\map \cos {a + b} = \cos a \cos b - \sin a \sin b$
 * $\map \cos {a - b} = \cos a \cos b + \sin a \sin b$

In the $6$th edition, the structure of the dictionary has changed to put these formulae into an appendix where they are specified in full, error-free.