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How to find a second derivative

how to find a second derivative

Example 1   Find  for .


There are actually two solution methods for this problem.

Solution 1 :

This is the simple way of doing the problem.  Just solve for y to get the function in the form that we’re used to dealing with and then differentiate.

So, that’s easy enough to do.  However, there are some functions for which this can’t be done.  That’s where the second solution technique comes into play.

Solution 2 :

In this case we’re going to leave the function in the form that we were given and work with it in that form.  However, let’s recall from the first part of this solution that if we could solve for y then we will get y as a function of x .  In other words, if we could solve for y (as we could in this case, but won’t always be able to do) we get .  Let’s rewrite the equation to note this.

Be careful here and note that when we write  we don’t mean y times x .  What we are noting here is that y is some (probably unknown) function of x .  This is important to recall when doing this solution technique.

The next step in this solution is to differentiate both sides with respect to x as follows,

The right side is easy.  It’s just the derivative of a constant.  The left side is also

easy, but we’ve got to recognize that we’ve actually got a product here, the x and the .  So to do the derivative of the left side we’ll need to do the product rule.  Doing this gives,

Now, recall that we have the following notational way of writing the derivative.

Using this we get the following,

Note that we dropped the  on the y as it was only there to remind us that the y was a function of x and now that we’ve taken the derivative it’s no longer really needed.  We just wanted it in the equation to recognize the product rule when we took the derivative.

So, let’s now recall just what were we after.  We were after the derivative, . and notice that there is now a  in the equation.  So, to get the derivative all that we need to do is solve the equation for .

There it is.  Using the second solution technique this is our answer.  This is not what we got from the first solution however.  Or at least it doesn’t look like the same derivative that we got from the first solution.  Recall however, that we really do know what y is in terms of x and if we plug that in we will get,

which is what we got from the first solution.  Regardless of the solution technique used we should get the same derivative.

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