Cosine of Sum/Mistake

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Source Work

2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.):

compound angle formulae (in trigonometry)


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


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:

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