m(∡AOB)+m(∡BOC)=180°
m(∡BOC)=120°⇒m(∡AOB)=180°-m(∡BOC)=180°-120°=60°
m(∡AOD)=130° ⇒
m(∡BOD)=m(∡AOD)-m(∡AOB)=130°-60°=70° ⇒
m(∡DOC)=m(∡BOC)-m(∡BOD)=120°-70°=50°
OM bisectoarea ∡AOD
m(∡AOM)=m(∡MOD)=130°:2=65°
⇒ m(∡BOM)=m(∡BOD)-m(∡MOD)=70°-65°=5°
ON bisectoarea ∡COD
m(∡CON)=m(∡NOD)=50°:2=25°
m(∡MON)= m(∡MOD)+ m(∡DON)=65°+25°=90°