A+B+C=180; B+C=180-A=180-54=> B+C=126 GRADE. Bisectoarea imparte unghiul in doua parti egale. fie tr BIC. ∡IBC+∡ICB+∡BIC=180. DAR ∡IBC=∡B/2 IAR ∡ICB=∡C/2=>B/2+C/2+∡BIC=180, REDUCEM NUMITORUL=>B+C+2*∡BIC=360 =>126+2*BIC=360 =>2*∡BIC=234 =>∡BIC=117 GRADE