答：
5a³-3b²-5a³+4b²+2ab,其中a＝-1,b＝1/2
=(5a³-5a³)+(-3b²+4b²)+2ab
=0+b²+2ab
=(b+2a)b
=(1/2 -2)*(1/2)
=(-3/2)*(1/2)
=-3/4先合并同类项，再求值对啊