The Mean Value Theorem
If đ is continuous on [đ, đ] and differentiable on (đ, đ), then there exists some đ with đ < đ < đ such that
đâ˛(đ) = [đ(đ) â đ(đ)] / (đ â đ)
This means the instantaneous rate of change at đ equals the average rate of change of đ over [đ, đ].