(1).（x+y)² -(x+y)三次方=(x+y)²[1-(x+y)]=(x+y)²(1-x-y)
(2).(x-y)² -(x-y)三次方=(x-y)²[1-(x-y)]=(x-y)²(1-x+y)
(3).(x-y)² -(y-x)三次方=(x-y)²+(x-y)三次方=(x-y)²[1+(x-y)]=(x-y)²(1+x-y)
(4).2(x-y) -3(y-x)²=2(x-y)-3(x-y)²=(x-y)[2-3(x-y)]=(x-y)(2-3x+3y)