题目

定义在上的函数满足（），，则等于（    ）A．2              B．3              C．6              D．9 答案：A  由条件f(x+y)=f(x)+f(y)+2xy对于x,y∈R恒成立.因此,f(0+0)=f(0)+f(0),得出f(0)=0.又因为f(1+1)=f(1)+f(1)+2,得出f(2)=2f(1)+2=6.又因为f(2+1)=f(2)+f(1)+4,得出f(3)=f(2)+f(1)+4=12,最后f［(-3)+3］=f(-3)+f(3)-18,得出f(0)=f(-3)+12-18,∴f(-3)=f(0)+6=6. The telephone _______ three times in the last hour, and each time it ________ for my father. A. had rang; was    B. has rung; was C. rang; has been       D. has been ringing; is