Proof en pudding

\( \eqalign{ & \frac{{\cos \left( u \right) + \sin \left( u \right)}} {{\cos \left( u \right) - \sin \left( u \right)}} = \frac{{1 + \sin (2u)}} {{\cos (2u)}} \cr & \frac{{\left( {\cos \left( u \right) + \sin \left( u \right)} \right)\left( {\cos \left( u \right) - \sin \left( u \right)} \right)}} {{\left( {\cos \left( u \right) - \sin \left( u \right)} \right)^2 }} = \frac{{1 + \sin (2u)}} {{\cos (2u)}} \cr & \frac{{\cos ^2 \left( u \right) - \sin ^2 \left( u \right)}} {{\cos ^2 (u) - 2\sin (u)\cos (u) + \sin ^2 (u)}} = \frac{{1 + \sin (2u)}} {{\cos (2u)}} \cr & \frac{{\cos \left( {2u} \right)}} {{1 - \sin (2u)}} = \frac{{1 + \sin (2u)}} {{\cos (2u)}} \cr & \cos ^2 (2u) = 1 - \sin ^2 (2u) \cr & \sin ^2 (2u) + \cos ^2 (2u) = 1 \cr & {\text{Klopt!}} \cr} \)