Factoriser
C(x)=5(2x+5)²-45
C(x)= 5(4x²+20x+25) -45
C(x)= 20x²+100x+125-45
C(x)= 20x²+100x+80
20x² + 100x + 80 = 0 avec a = 20 , b = 100 et c = 80
Δ = b² − 4ac = (100)² − 4×20×80 = 3600
Δ > 0 alors l'équation 20x² + 100x + 80 = 0 admet 2 solutions x1 et x2
or √3600 = 60
x1 = (-b − √Δ)/2a
x1= (-100 − 60) / 40
x1= -4
x2= (-b + √Δ)/2a
x2= (-100 + 60) / 40 = -1
20x² + 100x + 80 ⇒ 20(x + 4)(x + 1)
