DM pour demain aidez-moi svp!

thales, tu expliques pourquoi

AM/AB = AN/AC = MN/BC
AM/12 = 12/16
16*AM = 12*12
AM = 144/16 = 9cm

AN/AC = MN/BC
12/16 = 4,5/BC
12*BC = 16*4,5
BC  = 6cm