设f(x)=(y+1)/(x+2),x^2+y^2=1带入(y+1)/(x+2)
得f(x)=((1-x^2)^1/2+1)/(x+2),对f(x)求导,得f'(x)=0,求出x=0或-4/5
x=-4/5,f"(x)