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You guys aren't going to believe that I can get stuck on something this pathetically easy, but I have to ask anyway.

**Find two positive numbers whose product is 108 and the sum of the first number plus three times the second number is a minimum.**

Here is what I've done:

xy = 108

x + 3y = S

Let x and y be the two numbers, and S the sum.

To find S substitute x = (108/y) into x + 3y = S

S = (108/y) + 3y

Now I need to take the derivative of the function, and set it equal to zero to find a critical number, but I can't get the right answer for this part, which is stupid because it should be so darn easy. I haven't done derivatives in awhile and now I am starting to forget them

This is what the book says :

dS/dy = -(108/y^2) + 3 = 0

3 = 108/y^2

y^2 = 36

y = 6

The second derivative the book says is 216/y^3

Can someone please explain how they got the derivatives to me? I feel absolutely stupid having to ask this but I figured I had better ask now so I understand it later. Thanks so much