The Goal

Find

The Mistake

Find the mistake:

(Roll the mouse over the math to see a hint in red)

The Correction

(Roll the mouse over the area above to see the correction in blue)

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.