Các bước giải:
a/[(a+b)+c]²=a²+b²+c²+2ab+2cb+2ac
VT=[(a+b)+c]²
=(a+b)²+2ac+2bc+c²
=a²+2ab+b²+2ac+2bc+c²
=a²+b²+c²+2ab+2cb+2ac=VP(đpcm)
b/[(a+b)-c]²=a²+b²+c²+2ab-2cb-2ac
VT=[(a+b)-c]²
=(a+b)²-2ac-2bc+c²
=a²+2ab+b²-2ac-2bc+c²
=a²+b²+c²+2ab-2cb-2ac=VP(đpcm)