4/9是一个整体a平方-4a+9（2）（x+y）²-6x-6y+9（3）4（x+2y）²-25（x-y）²

4/9是一个整体a平方-4a+9
=4a^2/9-4a+9
=1/9(4a^2-36a+81)
=1/9(2a-9)^2

(x+y）²-6x-6y+9
=(x+y）²-6(x+y)+9
=(x+y-3)^2

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

2xy-x平方-y平方+1
=1-(x-y)^2
=(1+x-y)(1-x+y)

（9a²-4b²）/（3a-2b）
=(3a+2b)(3a-2b)/（3a-2b
=3a+2b

（2x²-2x+0.5）/（x-0.5)
=1/2(2x-1)^2/[1/2(2x-1)]
=2x-1