If you are talking about the identity given by dwsmith, you could find it in your precalc book or look online for properties of logarithms
Use the laws of logs to avoid having to mess with fractions.
You will still need to apply the Chain Rule
and use
Then
so
and so on......
Here's how to use the Chain Rule:
You've gotten to
The derivative "with respect to x" of is
We want to differentiate "with respect to x" however.
If we write then we can say we are calculating the derivative of "with respect to x".
However
But what about
We have to use the Chain Rule for that...
We multiply those 2 derivatives and we can calculate both of them.
Now we replace "u" with it's original expression in x.
Therefore
by multiplying the derivatives.
The same procedure applies to the 2nd log expression. Then the 2 results need to be subtracted.