bonjour,
40 = x+2y
si x = 4
40 = 4+2y
-2y = -4+40
y = 9
A = x*y = 4*9 = 36m²
si x =10 :
10+2y = 40
2y = 40-30
y = 5
A = 10*5 = 50m²
si x = 20
20+2y = 40
2y = 40-20
y = 10
A = 20*10 =200m²
si x = 28,
28+2y = 40
2y = 40-28
y = 6
A = 28*6 =168m²
2) 2y = 40-x
y = (40-x)/2 = 20-0,5x
A = y*x
y = 20-0,5x
A = (20-0,5x)x = 20x-0,5x²