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