（1）（x+y-3)(x+y+3)
=[(x+y)-3][(x+y)+3]
=(x+y)²-9
=x²+2xy+y²-9
(2)(x-1)(x+1)(x^2-1)
=(x²-1)²
=x的4次方-2x²+1
(3)(x+2y)^2+2(x+2y)+1
=(x+2y+1)²第三道题过程