Salut !
1) a) si x=20
aire ABCDEF = (40+20) × (20+120) - 40² - 20² = 6 400 m²
b) si x=30
aire ABCDEF = (40+30) × (30+120) - 40² - 30² = 8 000 m²
2) A = (40+x) × (120+x) - 40² - x²
3) (40+x) × (120+x) - 40² - x² = 4 800 + 40x + 120x + x² - 40² - x²
= 160x + 4 800 - 1 600
= 160x + 3 200
4) A = 160x + 3 200 = 160(x+20)