http://i.imgur.com/Hz4w1SI.png

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I don't understand how you get -(1/6)pi. It didn't really explain how that was calculated. I've been setting the neg square root of 3 over 3 equal to y/x, solving for y then pluging it in to the Pythagorean theorem, but it's not working out. Is this the correct way to solve this? Is anyone else about to get -1/6pi with this method?