c) goc ANB+goc ACB=goc ANB+goc ABC=180o-goc NEF+goc BNE+goc ABC=180o-goc NEF+goc NBC=180o-goc FEM+goc NBC=90o+goc NME+goc NBC
=360o-goc FEB-goc ABC-goc EMB+goc NBC=180o+goc AEF+goc NBA-goc EMB=180o+goc NAB+goc NBA-goc EMB
=180o+goc NAB+goc NBA-goc ABC=360o-goc ANB-gocABC
Ta co : goc ANB+goc ACB=goc ANB+goc ABC=360o-goc ANB-gocABC
=> goc ANB+goc ACB=180o