P - perimetru
L-lungime
l- latime
A- arie
P= 2L+2l=42 /:2
L+l=21
A=L*l
l=L-4
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transformi 400 cm in metri , 400cm= 4m
L+l=21
L+L-4=21
2L=25
L=12.5 m
l= 21-12.5
l=8.5m
A=L*l= 12.5*8.5 = 106.25 m^{2}
1 hectar(ha) = 10.000 m^{2}
1 ha = 100 ari
106,25 m^{2} = 0,01625 ha