g(x)=-x^2-3
f(x)=ax^2+bx+c
f+g=(a-1)x^2+bx+c-3
是奇函数,不存在二次项和常数项
a=1,c=3
f（x）=x^2+bx+3
=x^2+bx+(b/2)^2-(b/2)^2+3
=(x+b/2)^2-(b/2)^2+3
最小值为-(b/2)^2+3=1
b=±√8
f(x)=x^2±√8x+3