5.5 Differentiation of Logarithmic Functions

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5.5 Differentiation of Logarithmic Functions
By Dr. Julia Arnold and Ms. Karen Overman using Tan’s 5th edition Applied Calculus for the managerial , life, and social sciences text

Now we will find derivatives of logarithmic functions and we will
Need rules for finding their derivatives. Rule 3: Derivative of ln x Let’s see if we can discover why the rule is as above. First define the natural log function as follows: Now rewrite in exponential form: Now differentiate implicitly:

Example 1: Find the derivative of f(x)= xlnx.
Solution: This derivative will require the product rule. Product Rule: (1st)(derivative of 2nd) + (2nd)(derivative of 1st)

Example 2: Find the derivative of g(x)= lnx/x
Solution: This derivative will require the quotient rule. Quotient Rule: (bottom)(derivative of top) – (top)(derivative of bottom) (bottom)²

Why don’t you try one: Find the derivative of y = x²lnx .
The derivative will require you to use the product rule. Which of the following is the correct? y’ = 2 y’ = 2xlnx y’ = x + 2xlnx

No, sorry that is not the correct answer.
Keep in mind - Product Rule: (1st)(derivative of 2nd) + (2nd)(derivative of 1st) Try again. Return to previous slide.

Good work! Using the product rule:
F’(x) = (1st)(derivative of 2nd) + (2nd)(derivative of 1st) y’ = x² (lnx)(2x) y’ = x + 2xlnx This can also be written y’ = x(1+2lnx)

Here is the second rule for differentiating logarithmic functions.
Rule 4: The Chain Rule for Log Functions In words, the derivative of the natural log of f(x) is 1 over f(x) times the derivative of f(x) Or, the derivative of the natural log of f(x) is the derivative of f(x) over f(x)

Example 3: Find the derivative of
Solution: Using the chain rule for logarithmic functions. Derivative of the inside, x²+1 The inside, x²+1

Example 4: Differentiate
Solution: There are two ways to do this problem. One is easy and the other is more difficult. The difficult way:

The easy way requires that we simplify the log using some of the expansion properties.
Now using the simplified version of y we find y ’.

Now that you have a common denominator, combine into a single
fraction. You’ll notice this is the same as the first solution.

Example 6: Differentiate
Solution: Using what we learned in the previous example. Expand first: Now differentiate: Recall lnex=x

Find the derivative of . Following the method of the previous two examples. What is the next step?

This method of differentiating is valid, but it is the more difficult
way to find the derivative. It would be simplier to expand first using properties of logs and then find the derivative. Click and you will see the correct expansion followed by the derivative.

Correct. First you should expand to Then find the derivative using the rule 4 on each logarithm. Now get a common denominator and simplify.

Example 7: Differentiate
Solution: Although this problem could be easily done by multiplying the expression out, I would like to introduce to you a technique which you can use when the expression is a lot more complicated. Step 1 Take the ln of both sides. Step 2 Expand the complicated side. Step 3 Differentiate both side (implicitly for ln y )

Step 4: Solve for y ‘. Step 5:Substitute y in the above equation and simplify.

Continue to simplify…

Let’s double check to make sure that derivative is correct by
Multiplying out the original and then taking the derivative. Remember this problem was to practice the technique. You would not use it on something this simple.

Consider the function y = xx.
Not a power function nor an exponential function. This is the graph: domain x > 0 What is that minimum point? Recall to find a minimum, we need to find the first derivative, find the critical numbers and use either the First Derivative Test or the Second Derivative Test to determine the extrema.

To find the derivative of y = xx , we will take the ln of both sides first and then expand.
Now, to find the derivative we differentiate both sides implicitly.

To find the critical numbers, set y’ = 0 and solve for x.
Now test x = 0.1 in y’, y’(0.1) = < 0 and x = 0.5 in y’, y’(0.5) = > 0 Thus, the minimum point occurs at x = 1/e or about .37

We learned two rules for differentiating logarithmic functions:
Rule 3: Derivative of ln x Rule 4: The Chain Rule for Log Functions We also learned it can be beneficial to expand a logarithm before you take the derivative and that sometimes it is useful to take the natural log (ln) of both sides of an equation, rewrite and then take the derivative implicitly.