( a - 3 )" + ( a - 3 )
= ( a - 3 )( a - 3 + 1 )
= ( a - 3 )( a - 2 )
或者
= a" - 6a + 9 + a - 3
= a" - 5a + 6
= a" - 2a - 3a + 6
= a( a - 2 ) - 3( a - 2 )
= ( a - 2 )( a - 3 )