(a-2b+3c)²-(-a+2b-3c)²
=[(a-2b+3c)+(-a+2b-3c)][(a-2b+3c)-(-a+2b-3c)]
=0x[(a-2b+3c)-(-a+2b-3c)]
=0=[(a-2b+3c)+(-a+2b-3c)][(a-2b+3c)-(-a+2b-3c)]为什么？利用的是平方差公式a²-b²=(a+b)(a-b)