Bonjour,
1) A(x) = (3x+1)²-(3x+1)(7x-4)
= 9x²+6x+1-(21x²-12x+7x-4)
= 9x²-21x²+6x+12x-7x+1+4
= -12x²+11x+5
2) A(x) = (3x+1)²-(3x+1)(7x-4)
= (3x+1)(3x+1)-(3x+1)(7x-4)
= (3x+1)[(3x+1)-(7x-4)]
= (3x+1)(-4x+5)
= -(3x+1)(4x-5)
3) A(0) = -12(0²)+11(0)+5 = 5
A(-1/3) = -(3(-1/3)+1)(4(-1/3)-5) = 0 car (3(-1/3)+1)=0
4) A(x) = 0 ⇒ -(3x+1)(4x-5) = 0
⇒ 3x+1=0 ou 4x-5=0
⇒ 3x=-1 ou 4x=5
⇒ x = -1/3 ou x = 5/4
