Répondre :
A=3(x-10)(x+10)+(2x+5) (x+10)
= 3(x² - 10²) + 2x² + 20x + 5x + 50
= 3x² - 300 + 2x² + 25x + 50
= 5x² +25x - 250
B=4(3x+6)(5x+1)-2(3x-6)²
= (12x+24)(5x+1)-(12x-24)²
= 60x² + 12x + 120x + 24 - 12x² + 576x - 576
= 48x² + 708x - 552
C= 4(7x+5)²+4(2x-1)(7x+5)
= (112x + 80)² + (8x - 4)(7x+5)
= 12544x² + 17920x + 6400 + 56x² + 40x - 28x - 20
= 12600x² + 17932x + 6380
En espérant t'avoir aidé.