设全集U={1,2,3,4,5},集合M满足CUM={1,3},则
(I) 求曲线y= f(x)在点(0,f(0))处的切线方程;
(II)设g(x)= f"(x),讨论函数g(x)在[0,+∞).上的单调性;
(II)证明:对任意的s,t∈(0,+∞),有f(s+t)> f(s)+ f(t).