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Chain Rule: The General Logarithm Rule - Problem 1 3,968 views

Teacher/Instructor Norm Prokup
Norm Prokup

Cornell University
PhD. in Mathematics

Norm was 4th at the 2004 USA Weightlifting Nationals! He still trains and competes occasionally, despite his busy schedule.

You can use the chain rule to find the derivative of a composite function involving natural logs, as well. Recall that the derivative of ln(x) is 1/x. For example, say f(x)=ln(g(x)), where g(x) is some other function of x. By the chain rule, take the derivative of the "outside" function and multiply it by the derivative of the "inside" function. With the derivative of logarithmic functions, the outside function is the logarithm itself, and the inside function is what is inside the logarithm. So, f'(x)=1/g(x) * g'(x).

We’re talking about how to differentiate a special kind of composite function. This kind; natural log of g(x). Now because the derivative of natural log is 1 over x, the derivative quantity of the chain, would be 1 over g(x) times g'(x). So let’s differentiate one of these kinds of functions. H(x) equals natural log of 1 plus e to the x.

I have it written up here; h' is going to be 1 over the inside stuff, 1 over 1 plus e to the x times the derivative, with respect to x of 1 plus e to the x. The derivative of the inside. Of course that’s just going to be zero plus e to the x. So 1 plus e to the x here. And that’s just e to the x over 1 plus e to the x. that’s the derivative of natural log of 1 plus e to the x.

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