Bonjour
1) Développer et réduire A
A = x²-25 - (x+5) (4-3x)
A = x² - 25 - (4x - 3x² + 20 - 15x)
A = x² - 25 - 4x + 3x² - 20 + 15x
A = x² + 3x² - 4x + 15x - 25 - 20
A = 4x² + 11x - 45
2) Factoriser x² -25 puis factoriser A
A = x²-25 - (x+5) (4-3x)
A = (x + 5) (x + 5) - (x + 5) (4 - 3x)
A = (x + 5) [(x - 5) - (4 - 3x)]
A = (x + 5) (x - 5 - 4 + 3x)
A = (x + 5) (4x - 9)
3)Calculer A lorsque x = 0
A = (x + 5) (4x - 9)
(x + 5) (4x - 9) = 0
x + 5 = 0 ou 4x - 9 = 0
x = 5 4x = 9
x = 9/4.
