1.Fie ABCD romb
P=4l⇒20=4l⇒l=20/4⇒l=5cm
BD intersectat cu AC={O}⇒BO=BD/2=6/2=3cm
ΔAOB⇒T.P.⇒AO²=AB²-BO²
AO²=25-9
AO²=16
AO=4cm⇒AC=2AO=2*4=8cm
A romb=AC*BD/2=8*6/2=24cm²
2.ABCD trapez isoscel cuADintersectat cuBC={M}
DC=6cm ⇒DC=AB/2⇒DC l mijl inΔMAB (DC II AB) ⇒
AB=12 cm
⇒MD=MC=6cm⇒MA=MB=12cm⇒ΔMAB echilateral⇒P=3*12=36cm