(a+b)²≥0,要想(a+b)²+2取最小值时必须是(a+b)²取最小值,即
(a+b)²=0,所以a+b=0
a³b+(2a²b²-ab)-(5a²b²+4a³b-8)-a²
=a³b+2a²b²-ab-5a²b²－4a³b＋8-a²
=﹣3a³b－3a²b²－ab－a²＋8
=﹣3a²b(a＋b)－a(a＋b)＋8
=0－0＋8
=8