19.
AB=Paob - AC/2- BD/2= 28-6-10=12
BC=Pboc - BD/2- AC/2 = 25-10-6=9
Pabcd = 2x12+ 2x 9 = 42
20.
Pabcd =40
2L+2l = 40 (1)
Paob =24
L+d/2+D/2=24 (2)
Pboc = 20
l+d/2+D/2 = 20 (3)
(2) - (3) ⇒ L- l =24 -20 =4 ⇒ L=l+4 inlocuim in (1)
2(l+4)+2l=40
2l+8+2l=40
4l=40-8
4l = 32
l = 8
L =l+4 =8+4 = 12
