## Confounding Calculus

November 15, 2012 by richbeveridge

Another fun Calculus problem we ran into the other day involved using the quotient rule versus using the product rule. This problem also came from the Briggs and Cochran book in the section on Implicit Differentiation (pg. 162 #15). So, the problem this time started out as

Use implicit differentiation to find .

So, if we start out by differentiating the expressions on each side, we get

If we expand and simplify a little –

So then

and

This is the answer that appears in the text – but, if we assume that , then we can clear the denominator in the original problem and use implicit differentiation on the result.

Then, differentiating

or

Then, to isolate :

and

I spent some time playing around with these, and believe me, they are not the same. I graphed them and here is what the first answer looks like:

And here is the second version,

I tried plugging values for and into these different expressions for the derivative of the same function to see if I got the same answer. This led me to realize the solution to the conundrum!

I started by plugging in some simple values for and like but quickly realized that this wasn’t valid because . So then I tried and for these values, the two expressions were the same!

and

These are the same – because the point (1,0) satisfies the original function: .

So I got out the Excel spreadsheet and found a series of points that satisfied the original function and tried a few of these points in the two expressions for the derivative and for each one the value of the two expressions is the same!

So next I was determined to show that assuming the original function was true would show the equality of the two different expressions for the derivative.

So, assuming we need to turn into .

First I substituted in for , so:

or

Then I cleared the fraction in the numerator and denominator:

OK, so this is

or

I saw that where I was headed with this had a and a in it, so I moved a few things around and ended up with:

Going back to the original function I saw that I could substitute in for the :

Factor out from the everything except the and this is starting to look good:

And there it is:

– sigh of relief –

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