My lips are sealed, unless we're allowed to post our guess in a 'spoiler' kind of tag? Unvisible unless selected.
Perhaps another one, spot the error but don't reveal (or reveal covered using white text in a code box for example).
If you can spot the error please do not post the explanation let other people have fun solving it. For the first few days. If you do and I see it I will delete it.
Notice that where appears times. Thus, taking the derivative of both sides,
but 1 appears times thus, ? What is wrong?