# Use the first and second derivatives of the function y = x^3-3x^2-9x+15 to find the intervals of increase and decrease. Help!?

##### 1 Answer

The function is decreasing for

#### Explanation:

You may benefit from looking at some videos where the derivatives are explained, for instance

It is always a good idea to start with drawing a graph of your 3rd degree function (I've used Geogebra):

The graph shows that for

Let's use the 1st and 2nd derivatives to find this, as requested in the task:

The first derivative

We see that

Next: the 2nd derivative

A negative value tells us that the slope is changing from an increase to decrease

A positive value tells us that the slope is changing from a decrease to increase.

This is actually in agreement with what we could see from the graph.