Use the Envelope Theorem in this optimization question?

See my answer a href="http://answers.yahoo.com/question/index?qid=20110324142310AAyDDsE"http://answers.yahoo.com/question/index?…/a or read the wiki article on envelope thrm a href="http://en.wikipedia.org/wiki/Envelope_theorem" rel="nofollow"http://en.wikipedia.org/wiki/Envelope_th…/a