For any function [math]f\left(x\right)[/math] continuous on [math]\left[a,b\right][/math] and differentiable on [math]\left(a,b\right)[/math] there exists [br]some point [math]c[/math] in the interval [math]\left(a,b\right)[/math] such that the tangent at [math]c[/math] is parallel to the secant joining [br]the endpoints [math]\left(a,f\left(a\right)\right)[/math] and [math]\left(b,f\left(b\right)\right)[/math].