L=4l
P=l+l+L+L=820
2l+2L=820 /:2
l+L=410
l+4l=410
5l=410
l=410:5
l=82
L=82*4
L=328
VErificare:328+328+82+82=820 Adevarat
=====================================================
a, b, c - laturi
a=b=c
semiperimetru=a+b+c/2=300
a+b+c=600
a+a+a=600
3a=600
a=600:3
a=200 => b=200 => c=200