9(a-b)^2+12(a^2-b^2)+4(a+b)^2=[3(a-b)]^2+2×[3(a-b)×2(a-b)]+[2(a+b)]^2=[3(a-b)+2(a+b)]²=(5a-b)² 满意请采纳，O(∩_∩)O谢谢~
9(a-b)^2+12(a^2-b^2)+4(a+b)^2＝[（3（a-b)+2(a+b)]^2=(5a-b)^2
