Suppose f(x) is greater than or equal to 0 in the centripetal neighborhood of x0, but lim (x->; x0)f(x)= B& lt； 0

Proved by Theorem 3, the method is similar.

It is known that ε= | B |/2 exists in the centripetal neighborhood of x0, where | f (x)-b | < |B|/2.

Then there is f (x)

It is inconsistent with the assumption that f(x) is greater than or equal to 0 in the centripetal neighborhood of x0.