P(dreptunghi)=2×(L + l) stiim ca l / L = 2/5 adica: l = 2L / 5 !
⇵
2×(L + 2L/5) = 40 (cm)
L + 2L/5 = 40:2 = 20 | ×5
5L + 2L = 100
7L = 100 (cm) adica: L = 100 / 7
l = 2/5×100/7 = 2×20/7 = 40/7
( la 5 si 100 , am simplificat cu 5 ! )
A(dreptunghi) = L× l = 100/7cm × 40/7cm = 4000/49 ≈ 81,6cm²