1) a. AB+BC-DC+DA=0
VT=(AB+BC)+(DA-DC)
=AC+CA=AA=0(dpcm)
b. AB+CD=AD-BC
⇔AB+CD+DA+BC =0
VT=(AB+BC)+(CD+DA)=AC+CA=AA=0
2) a. OA+OB+OC+OD=0
VT=(OA+OB)+(OC+OD)
=AB+CD=AB+BA(CD=BA)=AA=0
b. DA-DB+OC=OD
⇔DA-DB+OC-OD=0
VT=(DA-DB)+(OC-OD)=BA+DC=BA+AB(DC=AB)=0
c.DA-NB+ND+DC=0
VT=(DA+DC)+(ND-NB)=AC+BD=DB+BD(AC=DB)=DD=0
CHO MH 5* NHÉ@-@