Bonsoir
A = (3x + 1)²
A = (3x + 1) (3x + 1)
A = 3x × 3x + 3x × 1 + 1 × 3x + 1 × 1
A = 9x² + 3x + 3x + 1
A = 9x² + 9x + 1
B = (7 - x)²
B = (7 - x) (7 - x)
B = 7 × 7 - 7 × (-x) - x × 7 - x × (-x)
B = 49 + 7x - 7x + x²
B = 49 + x²
C = (9 - 5x) (9 + 5x)
C = 9 × 9 - 9 × 5x - 5x × 9 + 5x × 5x
C = 81 - 45x - 45x + 25x²
C = 81 - 25x²