Condition for increasing decreasing and concavity of function

If the concavity of f changes at a point (c,f(c)), then f′ is changing from increasing to decreasing (or, decreasing to increasing) at x=c. That means that the sign of f″ is changing from positive to negative (or, negative to positive) at x=c. This leads to the following theorem

Step-by-step explanation:

The previous section showed how the first derivative of a function,  f′ , can relay important information about  f . We now apply the same technique to  f′  itself, and learn what this tells us about  f . The key to studying  f′  is to consider its derivative, namely  f′′ , which is the second derivative of  f . When  f′′>0 ,  f′  is increasing. When  f′′<0 ,  f′  is decreasing.  f′  has relative maxima and minima where  f′′=0  or is undefined. This section explores how knowing information about  f′′

Let  f  be differentiable on an interval  I . The graph of  f  is concave up on  I  if  f′  is increasing. The graph of  f  is concave down on  I  if  f′  is decreasing. If  f′  is constant then the graph of  f  is said to have no concavity.

Note: We often state that " f  is concave up" instead of "the graph of  f  is concave up" for simplicity.

The graph of a function  f  is concave up when  f′  is increasing. That means as one looks at a concave up graph from left to right, the slopes of the tangent lines will be increasing. Consider Figure  3.4.1 , where a concave up graph is shown along with some tangent lines. Notice how the tangent line on the left is steep, downward, corresponding to a small value of  f′ . On the right, the tangent line is steep, upward, corresponding to a large value of  f′ .