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

The derivative of 400r (with respect to r) is 400, not r.

(Multiplying out before taking the derivative was a good idea, since it avoided using the product rule, which would have required more work for this derivative.)