dy
that's twice ...
http://www.mathhelpforum.com/math-he...ed-please.html
Just in case a picture helps ...
(and helps with the checking by differentiating, too... check again!)
As usual, straight continuous lines differentiate downwards with respect to y, and the straight dashed line with respect to the dashed balloon expression - so that the triangular network is the chain rule.
Don't integrate - balloontegrate!
http://www.ballooncalculus.org/forum.top.php