-0.2x² + 0.8x + 15.4 = 0 ; a= -0.2 , b= 0.8 et c =15.4
Δ = b² − 4ac = (0.8)²− 4(-0.2)(15.4) = 12.96
Δ > 0 2 solutions réelles x1 et x2
x1 = (-b − vΔ)/2a = [-0.8 − v(12.96)] / (-0.4) = 11
x2 = (-b + vΔ)/2a = [-0.8 + v(12.96)] / (-0.4)= -7
S= [-7 ; 11]