令x=y=0 则f(0)=f(0)+f(0)+2所以f(0)=-2令y=-x则f(0)=f(x)+f(-x)+2所以f(x)+f(-x)=-4
f(x)+f(-x)+2=f(x-x)=f(0)f(0)+f(0)+2=f(0+0)=f(0)，所以，f(0)=-2所以，f(x)+f(-x)=-2-2=-4
