Neka je
![](https://static.elitesecurity.org/tex/d658f65cba35c3be36b4aee61b82bf1b.png)
pramen pravih i
![](https://static.elitesecurity.org/tex/7d5b1ffb5acd6ee28755d9288a42ff95.png)
proizvoljna tacka u ravni
![](https://static.elitesecurity.org/tex/3e25754f2397ae263143bda67ff13610.png)
tog pramena koja ne pripada svim pravama pramena
![](https://static.elitesecurity.org/tex/d658f65cba35c3be36b4aee61b82bf1b.png)
. Skup svih tacaka ravni
![](https://static.elitesecurity.org/tex/3e25754f2397ae263143bda67ff13610.png)
, osnosimetricnih tacki
![](https://static.elitesecurity.org/tex/7d5b1ffb5acd6ee28755d9288a42ff95.png)
u odnosu na prave pramena
![](https://static.elitesecurity.org/tex/d658f65cba35c3be36b4aee61b82bf1b.png)
cemo zvati
epicikom i obelezavacemo ga sa
![](https://static.elitesecurity.org/tex/50c011e4faff4427f4e2b9d65d1f70a5.png)
.
Ako je
![](https://static.elitesecurity.org/tex/3be1269ff3be323765a3474d14212995.png)
pramen konkurentnih pravih, epicikl
![](https://static.elitesecurity.org/tex/15ea36393e2745cb3048e95c69f2f69f.png)
cemo zvati
krugom. Tacku
![](https://static.elitesecurity.org/tex/114c56781da755b1ec92890b585ac1a7.png)
cemo zvati
sredistem ili
centrom toga
kruga, a duz
![](https://static.elitesecurity.org/tex/165eb4dc4b1f6a273e12d5593e120540.png)
njegovim
poluprecnikom ili
radijusom. Krug cemo obelezavati i sa
![](https://static.elitesecurity.org/tex/470a6afae61e4048403db4f7ecd24aa4.png)
U toj oznaci
![](https://static.elitesecurity.org/tex/114c56781da755b1ec92890b585ac1a7.png)
je srediste, a
![](https://static.elitesecurity.org/tex/ab5fa9653d72e5bccec067d0006c090b.png)
poluprecnik kruga
![](https://static.elitesecurity.org/tex/18941089b86f1608ce4eb26ad3d3919e.png)
. Neposredno se dokazuje da krug
![](https://static.elitesecurity.org/tex/15ea36393e2745cb3048e95c69f2f69f.png)
istovetan sa skupom svih tacaka
![](https://static.elitesecurity.org/tex/6c5efcb639f59e6009c7d6f767e736ef.png)
ravni kojoj pripada, takvih da je
![](https://static.elitesecurity.org/tex/0f1552b056b05d36ae86c04d72183fbe.png)
(str. 135)
................................................................................
................................................................................
Unijom kruga i njegove unutrasnjosti cemo zvati i
kruznom povrsi, a sam krug cemo ponekad zvati i
rubom te povrsi. (str. 138)
Zoran Lucic, Euklidska i hiperbolicka GEOMETRIJA