Réponses

Meilleure réponse !
2014-09-15T17:09:46+02:00
4x²-9
= (2x)²-(3)²     identité remarquable a²-b² = (a-b) (a+b)
= (2x-3) (2x+3)
on a donc 
D = (2x-3) (2x+3) + (2x-3) (5x+4)
D = (2x-3) (2x+3+5x+4)
D = (2x-3) (7x+7)

D = 0 = (2x-3) (7x+7)
2x-3 = 0 ou 7x+7 = 0
2x = 3 ou 7x = -7
x = 3/2 ou x =-7/7
x = 3/2 ou x = -1

D = -21 = (2x-3) (7x+7)
14x²+14x-21x-21 = -21
14x²-7x = -21+21
14x²-7x = 0
7x(2x-1) = 0
7x = 0 ou 2x-1 = 0
x = 0 ou 2x = 1
x = 0 ou x = 1/2