（a-2）＾2-（2a-4）
=(a＾2-4a+4)-2a+4
=a＾2-4a+4-2a+4
=a＾2-6a+8
=a＾2-6a+9-1
=(a-3)＾2-1
=(a-3+1)(a-3-1)
=(a-2)(a-4)