( a + 2 )( a - 2 ) + 3a
= a" - 4 + 3a
= a" + 4a - a - 4
= a( a + 4 ) - ( a + 4 )
= ( a + 4 )( a - 1 )
或者
= a" - a + 4a - 4
= a( a - 1 ) + 4( a - 1 )
= ( a - 1 )( a + 4 )