a-b=8
a=b+8
代入ab+c^2+16=0
(b+8)b+c^2+16=0
b^2+8b+c^2+16=0
(b+4)^2+c^2=0
则b=-4,c=0
a=b+8=4
即a+b+c=4-4+0=0