解:原式=2x²-1/2+3x-4x+4x²-2 =6x²-x-5/2 当x=-1/2时, 原式=6×(-1/2)²-(-1/2)-5/2 =6×1/4+1/2-5/2 =3/2+1/2-5/2 =-1/2