Have you tried working through the derivatives one at a time?
The formula is a shorter way of doing the derivatives. I think you should work through them one at a time, in order to get a better understanding of the method.
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"What if you just eat vegetarians?"-neonreaper "^ You are what you eat. ^_~"-Koiji
Could try using wolfram alpha. That's usually my get out of hw hell free card. Haven't gotten up to that section yet myself so I can't really offer any other advice!