La dreptunghi laturile merg 2 cate 2 (congruente intre ele)
Fie x - lungimea si y - latimea
x+x+y=50dm
y+y+x=40dm
2x+y=50dm
2y+x=40dm
deci, scriem in loc de y, 50 dm -2x
deoarece suma - termen = termen
y=50dm-2x
2(50dm-2x)+x=40dm
deschidem parantezele
y=50dm-2x
100dm-3x=40dm
aflam 3x din aceasta
3x=60dm
x=20dm
aflam y din formula folosita
y=50dm-2x
y=50dm-40dm=10dm
x=20dm
y=10dm