5a³－（3b²＋5a³）＋（4b²+2ab）=b²+2ab当a=-1,b=1/2时，原式=1/4-1=-3/4
5a³－（3b²＋5a³）＋（4b²+2ab）=5a³－3b²-5a³＋4b²+2ab=5a³-5a³＋4b²－3b²+2ab=b²+2ab=(1/2)²+2*(-1)*1/2=1/4-1=-3/4
