1. (x-y)(x-y-3)+2
2. a平方-b平方+m平方-n平方+2(am-bn)
=(x-y)²-3(x-y)+2
=(x-y-1)(x-y-2)
=a²+2am+m²-b²-2bn-n²
=(a+m)²-(b+n)²
=(a+m+b+n)(a+m-b-n)