(1)f′(x)=3x2-2ax-1,
∵f(x)在(−
1
3,1)上单调递减,在(1,+∞)上单调递增,
∴f′(1)=0,
∴a=1;
(2)证明:当a=
1
2时,g(x)=f(x)-x+[3/2]=x3-[1/2]x2-2x+[3/2],
则g′(x)=3x2-x-2=(x-1)(3x+2),
∴0<x<1时,g′(x)<0,x>1时,g′(x)>0,
∴x=1时,g(x)min=g(1)=0,
∴当x>0时,g(x)≥0,
∴当x>0时,f(x)≥x−
3
2.
(1)f′(x)=3x2-2ax-1,
∵f(x)在(−
1
3,1)上单调递减,在(1,+∞)上单调递增,
∴f′(1)=0,
∴a=1;
(2)证明:当a=
1
2时,g(x)=f(x)-x+[3/2]=x3-[1/2]x2-2x+[3/2],
则g′(x)=3x2-x-2=(x-1)(3x+2),
∴0<x<1时,g′(x)<0,x>1时,g′(x)>0,
∴x=1时,g(x)min=g(1)=0,
∴当x>0时,g(x)≥0,
∴当x>0时,f(x)≥x−
3
2.