1)f'(x)=3x^2-6ax+3b=3(x^2-2ax+b)=2[(x-a)^2+b-a^2]
a0,当x>0,f'(x)>=b>0
a>0时,当x>0时,f'(x)的最小值为f(a)=b-a^2>=0---> a