| + | \(\lim _{ b\rightarrow a }{ \frac { f\left( b \right) -f\left( a \right) }{ b-a } } =f'\left( a \right) \\ \\ \forall \varepsilon >0\quad \exists \delta >0\quad (\delta >\left| b-a \right| >0\quad \Rightarrow \quad \left| \frac { f\left( b \right) -f\left( a \right) }{ b-a } -f'\left( a \right) \right| <\varepsilon \quad )\\ \quad \Leftrightarrow \quad \forall \varepsilon >0\quad \exists \delta >0\quad (\quad (a-\delta <b<a+\delta \quad \wedge \quad b≠a)\quad \Rightarrow \quad f'\left( a \right) -\varepsilon <\frac { f\left( b \right) -f\left( a \right) }{ b-a } <f'\left( a \right) +\varepsilon \quad )\quad ) \) |