Perim ACD=AC+CD+DA=11
Perim BCD=BC+CD+BD=9
dac le adunam obtinem
AC+CD+DA+BC+CD+BD=2CD +AC + BC+BD+DA=
2CD+AC +BC+BA= 2CD+ Perim ABC
Adica
9+11=2CD+14
20=2CD+14
2CD=6
CD=3
Veziu in desen cumprin adunarea a doua perimetre se obtine al treila, ABC si , in plus, 2CD