bonjour

largeur =  x

2 x + 20 = 31

2 x = 31 - 20

2 x = 11

x = 5.5

largeur  = 5.5 cm

5.5 * 10 = 55

aire = 55 cm²

f (x) =  15.5 x - x²

f ( 0 ) = 0

f ( 4 ) = 62 - 16 =  46

f ( 8.5) = 131.75 - 72.25 =  59.5

f ( 10 ) = 155 - 100 = 55

f ( 11) = 170.5 - 121 =  49.5

f ( 15.5 ) = 240.25 - 240.25 = 0

x (15.5 - x ) = 0

x = 0 ou 15.5