SOLUTION: Find (nth) derivative of f(x)=(ln(x))^2

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Question 1205160: Find (nth) derivative of f(x)=(ln(x))^2
Answer by Edwin McCravy(20056) About Me  (Show Source):
You can put this solution on YOUR website!
Sorry I'm just now posting this solution, but it just occurred to me
how to do it.

Let's take the first 3 derivatives.  I'm not going to
show my work to get the derivatives.  The first one we use
the power formula and the natural log formula. After that,
we just use the natural log formula, and the quotient formula.

f%28x%29%22%22=%22%22%28ln%28x%29%5E%22%22%29%5E2

d%2F%28dx%29f%28x%29%22%22=%22%22%282%2Aln%28x%29%29%2Fx

d%5E2%2F%28dx%5E2%29f%28x%29%22%22=%22%22%282-2%2Aln%28x%29%29%2Fx%5E2

d%5E3%2F%28dx%5E3%29f%28x%29%22%22=%22%22%284%2Aln%28x%29-6%29%2Fx%5E3

Let's write those derivatives in like form:

d%2F%28dx%29f%28x%29%22%22=%22%22+%28-1%29%5E1%2A%28%280-2%2Aln%28x%29%29%2Fx%5E1%29

d%5E2%2F%28dx%5E2%29f%28x%29%22%22=%22%22+%28-1%29%5E2%2A%28%282-2%2Aln%28x%29%29%2Fx%5E2%29

d%5E3%2F%28dx%5E3%29f%28x%29%22%22=%22%22+%28-1%29%5E3%2A%28%286-4%2Aln%28x%29%29%2Fx%5E3%29

Assume the (n-1)st derivative is of the form

d%5E%28n-1%29%2F%28dx%5E%28n-1%29%29f%28x%29%22%22=%22%22%28-1%29%5E%28n-1%29%2A%28a%5Bn-1%5D-b%5Bn-1%5D%2Aln%28x%29%29%2Fx%5E%28n-1%29

Then the nth derivative is

d%5En%2F%28dx%5En%29f%28x%29%22%22=%22%22

where the sequence {bn} is defined by 

b%5Bn%5D%22%22=%22%22b%5Bn-1%5D%2A%28n-1%29 where b%5B1%5D=2

and where the sequence {an} is defined by

a%5Bn%5D%22%22=%22%22a%5Bn-1%5D%2A%28n-1%29%2Bb%5Bn-1%5D where a%5B1%5D=0

Edwin