由图象过原点,得f(0)=d=0
f'(x)=3ax²+2bx+c,
f'(1)=0,得 3a+2b+c=0 (1)
f'(3)=0,得 27a+6b+c=0 (2)
又 f(1)=a+b+c=4 (3)
解得,a=1,b=-6,c=9
f(x)=x³-6x²+9x