a) vtAB + vtCD + vtEA = vtAC + vtCB + vtCD + vtED + vtDA
= (vtAC + vtCD + vtDA) + vtCB + vtED = vt0 + vtCB + vtED = vtCB + vtED => đpcm
b) vtAD + vtBE + vtCF = vtAE + vtED + vtBF + vtFE + vtCD + vtDF
= (vtED + vtFE + vtDF) + vtAE + vtBF + vtCD = vt0 + vtAE + vtBF + vtCD => đpcm
c) vtAB + vtCD + vtEF + vtGA = vtAC + vtCB + vtCE + vtED + vtEF + vtGF + vtFA
= (vtAC + vtCE + vtEF + vtFA) + vtCB + vtED + vtGF = vt0 + vtCB + vtED + vtGF => đpcm
d) vtAB - vtAF + vtCD - vtCB + vtEF - vtED
= vtAB + vtFA + vtCD + vtBC + vtEF + vtDE
= vtFB + vtBD + vtDF = vt0 => đpcm