1.（a-b+2)（a+b-2)
=[a-(b-2)][a+(b-2)]
=a²-(b-2)²
=a²-b²+4b-4
2.（x-1)²（x+1)²（x²+1）²
=(x²-1)²(x²+1)²
=(x^4-1)²
=x^8-2x^4+1
3.(2x+y-1）²
=[(2x+y)-1]²
=(2x+y)²-2(2x+y)+1²
=4x²+4xy+y²-4x-2y+1