[（a+b）（a-b）+（a-b）²+4a²（a+1）]÷a
=[a²-b²+a²-2ab+b²+4a³+4a²]÷a
=[4a³+6a²-2ab]÷a
=2（2a²+3a-b）．
当2a²+3a-b=4时，2（2a²+3a-b）=2×4=8．