**Derivative of a Function at \(a\)**, if it exists:

\[f'\left( a \right) = \mathop {\lim }\limits_{h \to 0} \frac{{f\left( {a + h} \right) - f\left( a \right)}}{h}\]

**Instructions:** Drag the green dot representing *a+h* toward *a* to simulate *h* tending toward zero.