# What is the formal definition of a derivative?

##### 1 Answer

May 29, 2015

The formal definition of derivative of a function

The meaning of this is best understood observing the following diagram:

The secant PQ represents the mean rate of change

If you want the rate of change, say, at P you "move" point Q (and the secant with it) to meet point P as in:

In doing so you must reduce

Hope it helps!