Scheitelwinkel sind gleich.

Nebenwinkel ergänzen sich zu ${180}^{\circ }180^\backslash circ$.

$\begin{array}{c}\alpha +{30}^{\circ }+(\alpha +{10}^{\circ })={90}^{\circ }\\ 2\alpha +{40}^{\circ }={90}^{\circ }\\ \alpha =\frac{{50}^{\circ }}{2}={25}^{\circ }\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash alpha+30^\backslash circ+(\backslash alpha+10^\backslash circ)=90^\backslash circ\backslash \backslash 2\backslash alpha+40^\backslash circ=90^\backslash circ\backslash \backslash \backslash alpha=\backslash frac\{50^\backslash circ\}2=25^\backslash circ\backslash end\{array\}$

Das Winkelmaß von $\alpha ={25}^{\circ }\backslash alpha=25^\backslash circ$.