U četverokutu ABCDABCDABCD je ∡DBC=∡DCB=50°\measuredangle DBC = \measuredangle DCB = 50°∡DBC=∡DCB=50° i ∡DAB=∡ABC=∡BDC\measuredangle DAB = \measuredangle ABC = \measuredangle BDC∡DAB=∡ABC=∡BDC. Dokaži da je AC⊥BDAC \perp BDAC⊥BD.