Unghiul = reuniunea a doua semidrepte inchise ce au aceeasi origine.

                                                                 Unitatea de masura a unghiului = gradul sexagesimal .

Submultipli grad : minutul, secunda.

                                                                                     10=60’=3600”

        

Tipuri de unghiuri:

- adiacente = au acelasi varf si o latura comuna,iar celelalte doua laturi sunt in semiplane opuse fata de latura comuna

- opuse la virf =congruente doua cate doua –laturile unuia sunt prelungirile laturilor celuilalt dincolo de varful lor comun.

                                                                 - complementare=cand suma lor =900

- suplementare = cand suma lor =1800

                                                                 - alungit =1800,laturile lui sunt semidrepte opuse.                    

                                                                  - nul =00,laturile lui coincid.                                                     

                                                                 - in jurul unui punct =3600

                                                                 - ascutit <900

                                                                 - drept =900                                                                             4     3

                                                                 - obtuz >900                                                              1      2

                                                                 - alterne interne 1-7,2-8

                                                                 - alterne externe 3-5,4-6                                         8     7

                                                                 - corespondente 1-5,2-6,3-7,4-8;                           5     6