Question

若不等式x^2+2x+a>= -y^2

Answer 1

x^2+2x+a>= -y^2-2y

x^2+2x+y^2+2y>=-a

x^2+2x+1+y^2+2y+1>=-a+2

(x+1)^2+(y+1)^2>=-a+2

因为(x+1)^2+(y+1)^2>=0

则只要-a+2