在x+2y-3=0上取一点,比如B(2, 1 2 )关于A的对称点C(m.n)在ax+4y+b=0上则BC中点是A m+2 2 =1, n+ 1 2 2 =0,解得:m=0,n=- 1 2 C(0,- 1 2 )所以0-2+b=0b=2故答案为:2