$sin (x_{1} \pm x_{2})$	$=$	$sin x_{1} \cdot cos x_{2} \pm cos x_{1} \cdot sin x_{2}$
$cos (x_{1} \pm x_{2})$	$=$	$cos x_{1} \cdot cos x_{2} \mp sin x_{1} \cdot sin x_{2}$
$tan (x_{1} \pm x_{2})$	$=$	$\frac{(tan x_{1} \pm tan x_{2})}{(1 \mp tan x_{1} \cdot tan x_{2})}$
Beispiel 11 - 1:
$sin 2 α$	$=$	$2 \cdot sin α \cdot cos α$
$cos 2 α$	$=$	${cos}^{2} α - {sin}^{2} α$
$\frac{1}{2} [1 - c o s (2 α)]$	$=$	$\frac{1}{2} [1 - {cos}^{2} α + s i n^{2} α]$
	$=$	$\frac{1}{2} [{sin}^{2} α + s i n^{2} α]$
	$=$	${sin}^{2} α$