原式=[3a+(b-2)][3a-(b-2)]
=(3a)²-(b-2)²
=9a²-b²+4b-4
原式=a+b-c）（a-b+c）-a²
=(a+(b-c))*(a-(b-c))-a²
=a²-(b-c)²-a2
=-b²+2bc-c²