bonjour
1) Périmètre = 2 L + 2 l
31 = 20 + 2 l
31 - 20 = 2 L
11 = 2 L
L = 5.5
BC = (31 - 2 x ) : 2
aire = x ( 31 - 2 x) : 2 = (31 x - 2 x² ) : 2 = x ( 15.5 - x )
f( 4) = 4 ( 15.5 - 4) = 4 *11.5 = 46
f(5) = 5 ( 15.5 - 5) = 5 * 10.5 = 52.5
x ( 15.5 x - x ) = 0
ou bien x = 0
ou bien x = - 15.5
