Vinkelsummen i en n-kant

$\begin{array}{rcl}\angle A& = & \angle C=50°\\ & & \\ \angle B& =& 180°-50°=130°\\ & & \\ \angle D& =& \angle B=130°\end{array}$

Vinkelsummen i en femkant er $3·180°$.

Vi finner

$\begin{array}{rcl}\angle B& = & 3·180°-102°-91°-127°-105°\\ & & \\ & =& 115°\end{array}$