bonsoir
longueur bande grise = 2 x + 3 - 2 = 2 x + 1
aire bande grise = ( 2 x + 1 )² = 4 x² + 4 x + 1
A = ( 2 x + 3 )² - ( 2 x + 1 )²
A = 4 x² + 12 x + 9 - ( 4 x² + 4 x + 1 )
A = 4 x² + 12 x + 9 - 4 x² - 4 x - 1
A = 8 x + 8
A = ( 2 x + 3 + 2 x - 1 ) ( 2 x + 3 - 2 x - 1 )
A = ( 5 x + 2 ) ( 2)