a" - a - 2
= a" - 2a + a - 2
= a( a - 2 ) + ( a - 2 )
= ( a - 2 )( a + 1 )
或者
= a" + a - 2a - 2
= a( a + 1 ) - 2( a + 1 )
= ( a + 1 )( a - 2 )