f(0)=f(0+0)=f(0)+f(0)+0 =2f(0)
所以f(0)=0
f(2)=f(1+1)=f(1)+f(1)+2 = 2+2+2=6
而0=f(0)=f(2-2)=f(2)+f(-2)-8=6+f(-2)-8=f(-2)-2
所以f(-2)=2