f.3因式分解~十萬火急!!20點!!

Question 4:

2(2x – y)^3 -4x [-1(2x –y)]^2
2(2x – y)^3 -4x [(-1)^2 (2x –y)^2 ]
2(2x – y)^3 -4x [1(2x –y)^2 ]
2(2x – y)^3 -4x (2x –y)^2
2(2x – y)^2[(2x – y) -2x]
2(2x – y)^2[2x – y -2x]
2(2x – y)^2[– y]
-2y(2x – y)^2

Check:
Let x = 2, y = 1
-2(1)(2(2) – 1)^2
-2(3)^2
-2(9)
-18

2(2x – y)^3 – 4x(y – 2x)^2
2(2(2) – 1)^3 – 4(2)(1 – 2(2))^2
2(3)^3 – 4(2)(-3)^2
2(27) – 4(2)(9)
54 – 72
-18

Both give a value -18. It is OK.