(1）(x+y)^2-6x-6y+9 =(x+y)^2-6(x+y)+9=(x+y-3)^2
（2）4(x+2y)^2-25(x-y)^2=(2(x+2y)-5(x-y))(2(x+2y)+5(x-y))=(9y-3x)(7x-y)=3(3y-1x)(7x-y)