8193: Differentiation - Trig. Functions - Visualise cos (x ± a) Addition Formulae

This display allows you to visualise the addition formulae:
$\color{purple}{ \cos (x + a) = \cos(x) \cos(a) - \sin(x) \sin(a) }$
$\color{purple}{ \cos (x - a) = \cos(x) \cos(a) + \sin(x) \sin(a) }$
 Select addition formula: