Bonjour !
1)
A = (5x - 7)(3x + 8)
= 5x*3x + 5x*8 - 7*3x - 7*8
= 15x² + 40x - 21x - 56
= 15x² + 19x - 56
B = (4x - 3)² + 10x(2x - 1)
= (4x)² - 2*4x*3 + 3² + 10x*2x + 10x*(-1)
= 16x² - 24x + 9 + 20x² - 10x
= 36x² - 34x + 9
C = (8x + 12)(8x - 12)
= (8x)² - 12² (3ème identité remarquable)
= 64x² - 144
2)
a) A_ABCD = L*l = (3x + 1)(3x - 4)
= 3x*3x - 4*3x + 1*3x - 1*4
= 9x² - 12x + 3x - 4
= 9x² - 9x - 4
b)
A(3.5) = 9*(3.5)² - 9*3.5 - 4
= 9*12.25 - 31.5 - 4
= 110.25 - 31.5 - 4
= 110.25 - 35.5
= 74.75 cm²
Voilà !
