The derivative of function _f_ at _x_ = _c_ is the limit of the slope of the secant line from _x_ = _c_ to _x_ = _c+h_ as _h_ approaches 0. Symbolically, this is the limit of [ _f_ ( _c_ )- _f_ ( _c_ + _h_ )]/ _h_ as _h_ ->0.
License: CC BY-NC-SA by Khan Academy