∡AOB + ∡BOC = 180;
∡MON = (∡AOB + ∡BOC ):2 = 180:2 = 90 grade;
a) Dacă ∡AOB = 60; ∡BOC = 120; ∡MON = (60 + 120):2 = 90 grade;
b) Dacă ∡BOC = 100; ∡AOB = 80; ∡MON = (100 + 80):2 = 90 grade;
c) ∡AOB=a grade; ∡BOC = (180 - a); ∡MON = (a - 180 + a):2 = 90;
2a = 270; a = 270:2 = 135; Deci ∡AOB = 135 grade; ∡BOC = 45 grade;
∡MON = 90 grade.