The Goal

Find

The Mistake

Find the mistake:

Need a hint? Look carefully at the red part:

The Correction

An Explanation

In the second factor the constant 200 has derivative 0, but the student left the 200 in place, perhaps distracted by differentiating the following term (1/2)r²π.

This derivative is easier to compute by multiplying out the expression to be differentiated first and then taking the derivative without using the product rule.