If the image of the function y=f(x) on the closed interval [a, b] is a continuous curve, and x. F (a). f (b)

~ Selected from the required course of mathematics in Interpretation and Expansion (Beijing Normal University Edition) 1.

Supplement: If the function y=f(x) is a continuous curve in the interval [a, b] and the sign of the function value at the end of the interval is opposite, that is, f (a). f (b)