Introduction to optimization

A farmer wants to plant a small garden along a river. He would like the garden to be rectangular and use the least amount of fencing possible. The area of the garden is $A$ square meters.

The area gives us this constrain equation:

$$A = xy$$The perimeter will be the objective function for this problem. However, note that one of the sides is bounded by a river, so the farmer doesn't need any fence on that side.$$f(x,y) = x + 2y$$

$x$ is the longest side and $y$ is the shortest. Or, expressing $y$ via $x$:$$f(x) = x + 2A/x$$

Now, select the option that will correctly find the first derivative of the objective function.