3(a方+b方+c方)=(a+b+c)方3a方+3b方+3c方=a方+b方+c方+2ab+2bc+2ca2a方+2b方+2c方-2ab-2bc-2ca=0(a-b)方+(b-c)方+(c-a)方=0又：(a-b)方≥0,(b-c)方≥0,(c-a)方≥0∴(a-b)方=(b-c)方=(c-a)方=0∴a-b=b-c=c-a=0∴a=b=c...