lpătrat = 21 dam
ldreptunghi = lp : 3
Pd = 2·Pp
~
Ldreptunghi = ?
~
Ppătrat = 4l
Ppătrat = 4 · 21 dam
Ppătrat = 84 dam
Pd = 2 · Pp
Pd = 2 · 84 dam
Pd = 168 dam
ldreptunghi = lp : 3
ld = 21 dam : 3
ld = 7
Pd = 2 · ( L + l )
168 = 2 · (L + 7)
2L + 14 = 168 /-14
2L = 154 /:2
L = 77
/robertinni