U konveksnom četverokutu ABCDABCDABCD vrijedi ∡BAD=50°\measuredangle BAD = 50°∡BAD=50°, ∡ADB=80°\measuredangle ADB = 80°∡ADB=80° i ∡ACB=40°\measuredangle ACB = 40°∡ACB=40°.
Ako je ∡DBC=30°+∡BDC\measuredangle DBC = 30° + \measuredangle BDC∡DBC=30°+∡BDC, izračunaj ∡BDC\measuredangle BDC∡BDC.