Réponse :

1)17) (5 - x)(-1 + x)

= -5 + 5x + x -x² = -x² + 6x -5

18) (x + 5)(3x - 1)

= 3x² -x + 15x -5 = 3x² +14x -5

19) (7x + 3)(x - 8)

= 7x² - 56x + 3x - 24 = 7x² -53x -24

2)17) a= 1 b=6 c=-5

Delta = b² - 4ac = 6² - 4 x -1 x (-5) = 36 - 20 = 16

x1 = (-b -(racine de delta)/2a = (-6 - (racine de 16))/2 = (-6 -4)/2 = -10/2 = -5

x2= (-b +(racine de delta)/2a = (-6 +(racine de 16))/2 = (-6 +4)/2 = -2/2 = -1

18) a= 3 b=14 c=-5

Delta = b² - 4ac = 14² - 4 x 3 x (-5) = 196 + 60 = 256

x1 = (-14 - (racine de 256))/2 = (-14 -16)/2 = -30/2 = -15

x2= (-14 +(racine de 256))/2 = (-14 +16)/2 = 2/2 = 1

19) a= 7 b=-53 c=-24

Delta = b² - 4ac = -53² - 4 x 7 x (-24) = 2809 + 672  = 3481

x1 = (53 - (racine de 3481))/2 = (53 - 59)/2 = -6/2 = -3

x2= (53 +(racine de 3481))/2 = (53 + 59)/2 = 112/2 = 56