知偶函数f(x)对任意x1,x2∈R,恒有f(x1+x2)=f(x1)+f(x2)+2x1x2+1

令x1=x2=0代入:f(0)=f(0)+f(0)+0+1,得：f(0)=-1
令x1=1,x2=-1代入,f(0)=f(1)+f(-1)-2+1=2f(1)-1,得：f(1)=0
令x1=x2=1代入:f(2)=f(1)+f(1)+2+1=2f(1)+3=3