Bonjour,
1)
A = 3x(2x - 5) - (x + 6)(2x - 5)
A = 3x × 2x + 3x × (-5) - (x × 2x + x × (-5) + 6 × 2x + 6 × (-5)
A = 6x² - 15x -2x² + 5x - 12x + 30
A = 4x² - 22x + 30
2)
A = 3x(2x - 5) - (x + 6)(2x - 5)
A = (2x - 5)[3x - (x + 6)]
A = (2x - 5)(3x - x - 6)
A = (2x - 5)(2x - 6)
3) Si x = 5 / 2
3 × 5 / 2(2 × 5 / 2 - 5) - (5 / 2 + 6)(2 × 5 / 2 - 5) = 0