Răspuns:
[tex] < a = \frac{ < b + < c + < d}{3} \\ \frac{ < b + < c + < d}{3} + < b + < c + < d = 360 \\ 4 < b + 4 < c + 4 < d = 1080 \\ se \: imparte \: relatia \: la \: 4 \\ < b + < c + < d = 270 \\ < a = \frac{270}{3} = 90 \: de \: unde \: rezulta \: ca \: paralelogramul \: este \: dreptunghi[/tex]