（1）（a-3b）^2
(2)(3m+4)^2
(3)(ab-1/2)^2
(4)(7x-y)^2
(5)(1/3+1/2 x^2)^2
(6)(5a-2b)^2
3a^2-12a^2+12a = -9a^2+12a= -3a(3a-4)
-9-6ab-a^2b^2 = -(a^2b^2+6ab+9)=-(ab+3)^2
1/4m^3n-1/4m^2n^2+1/16mn^3= 1/4mn(m^2-mn+1/4n^2)=1/4mn(m-1/2n)^2
a^4-2a^2+1 = (a^2-1)^2
(x-2y)^2-12(x-2y)+36 = (x-2y-6)^2
(a+2b)^2+2(a+2b)(2a+b)+(2a+b)^2 = (a+2b+2a+b)^2 = (3a+3b)^2=9(a+b)^2
基本上都是完全平方公式的运用~