xy(x-y)-x(x-y)^2
=x(x-y)[y-(x-y)]
=x(x-y)(y-x+y)
=x(x-y)(2y-x)
(a^2+b^2)^2-4a^2b^2
=(a^2+b^2-2ab) (a^2+b^2+2ab)
=(a-b)^2(a+b)^2
3、(36分之一a-三分之一)a+1
=(a/36-1/3)a+1
=a^2/36-a/3+1
=1/36(a^2-12a+36)
=1/36(a-6)^2
4、2(a-1)^2-12(a-1)+18
=2[(a-1)^2-6(a-1)+9]
=2(a-1-3)^2
=2(a-4)^2