As formula_21 approaches formula_25, formula_42 approaches formula_38. Therefore, formula_44. This proof can be generalised to show that formula_45 if formula_46 and formula_47 are constants. This is known as the power rule. For example, formula_48. However, many other functions cannot be differentiated as easily as polynomial functions, meaning that sometimes further techniques are needed to find the derivative of a function. These techniques include the chain rule, product rule, and quotient rule. Other functions cannot be differentiated at all, giving rise to the concept of differentiability.