bonsoir
soit AM = x
CB = 5
MB = 8 - x
aire CBM = [ 5 ( 8 - x ) ] / 2
= ( 40 - 5 x ) / 2 = 20 - 2.5 x
aire trapèze = [( 8 + x ) * 5 ]/ 2
= (40 + 5 x ) / 2 = 20 + 2.5 x
20 + 2.5 x = 3 ( 20 - 2.5 x )
20 + 2.5 x = 60 - 7.5 x
2.5 x + 7.5 x = 60 - 20
10 x = 40
x = 4
x ≥ 4