a²-b²-2b-1=a²-(b+1)²=(a+b+1)(a-b-1)
a²-b²-2b-1=a²-(b²+2b+1)=a²-(b+1)²=(a+b+1)(a-b-1)
a²-b²-2b-1=a²-（b²+2b+1）=a²-（b+1）²=（a-b-1）（a+b+1）
=a²-(b²+2b+1)=a²-(b+1)²=(a+b+1)(a-b-1)
