P = 2L + 2l = 2(L + l) = 28 dm
L = 2,5 * l
Il inlocuim pe L:
[tex]2(2.5l+l)=28\\
2( \frac{25}{10}l+ l )=28\\
2( \frac{5}{2}l+l )=28\\
2( \frac{5}{2} l+ \frac{2}{2}l )=28\\
2* \frac{7}{2}l=28\\
7l=28\\
l=4 dm\\
L=2.5*l=2.5*4=10 dm\\
A=L*l=40dm ^{2} [/tex]