In trapezul ABCD notam cu E si F mijloacele laturiilor neparalele [AD] , respectiv [BC] . Daca:
a) CD= 12 cm , EF=8 cm , aflati AB
b) AB= 9 cm , EF=21 cm , aflati CD
c) CD=35 cm , EF=28 cm , aflati AB
EF este linie mijlocie in trapez. Asta inseamna ca ef=(ab+cd)supra 2. Deci pt a) 8=(ab+12)/2=> 8x2=ab+12=>16=ab+12=>16-12=ab=>ab=4. Pt b) 21=(9+cd)/2=>21x2=9+cd=>42=9+cd=>cd=42-9=>cd=33. Pt c) 28=(35+ab)/2=>28x2=35+ab=>56=35+ab=>ab=56-35=>ab=21
