l= [tex] \frac{1}{3} * L[/tex]
P=l+l+L+L= 2*l+2L inlocuim l cu [tex] \frac{1}{3} * L[/tex]
P=2* [tex] \frac{1}{3} * L[/tex] + 2L =80 aducem la numitor comun
[tex] \frac{2L}{3} + \frac{6L}{3} = \frac{240}{3} [/tex] scapam de numitor si ne ramane
2L+6L = 240
8 L=240
L=240:8
L=30
l=[tex] \frac{1}{3} * L[/tex] = [tex] \frac{1}{3} *30 = \frac{30}{3} =10[/tex]
