(1) f(0)=c=0
f(1)=f(3)
-b/2=2,b=-4
(2) f(x)=x²-4x=(x-2)²-4
x∈(0 ,3)时,f(x)的最小值为f(2)=-4
f(x)的最大值为f(0)=0>f(3)=-3