In orice Δ avem :
masura unghiurilor este ∡A+∡B+∡C =180⁰
1/5 × A = 1/4 × B = 1/9 × C = k
1/5 × A = k ; A =5 k
1/4 × B = k ; B=4 k
1/9 × C =k ; C=9k
A+B+C=180
5k+4k+9k=180
18 k =180
k= 10
∡A= 5k = 5 × 10 = 50°
∡B= 4 k = 4 × 10 = 40°
∡C= 9 k = 9 × 10 = 90°
deci unghiurile triunghiului au masurile :∡A= 50° ; ∡B= 40° ; ∡C= 90°
verificare
∡A+∡B+∡C =180⁰
50+40+90=180
180 =180
