The Second Derivative Test is easy to apply once you know the second derivative, but unfortunately it does not always work. The First Derivative Test works more often, but is harder to apply. Compare the two tests by investigating the graphs of
The 1st and 2nd derivative tests are ways to tell what is going on at
Use the First Derivative Test to investigate the behavior of
, , and at the origin.
at two points, one to the left of and the other to the right of .
☛ If the sign changes from negative to positive, then
changes from decreasing to increasing and is a local min.
☛ If the sign changes from positive to negative, then
changes from increasing to decreasing and is a local max.
☛ If the sign does not change, then
is either increasing or decreasing (depending on the sign).
Try to confirm your results from part (a) by using the Second Derivative Test. When does the test work? What can you conclude if you know that the second derivative equals zero at the origin?
, then is concave up at , and is a local min.
, then is concave down at , and is a local max.
then this test is inconclusive.