f(x+y)=f(x)+f(y)+2xy
f(1+0)=f(1)+f(0)
f(1)=f(1)+f(0)
所以,f(0)=0
f(2)=f(1)+f(1)+2=4
f(3)=f(2+1)=f(2)+f(1)+4=4+1+4=9
f(4)=f(2+2)=f(2)+f(2)+8=16