行列式和叉積
http://www.evernote.com/shard/s146/sh/587e6520-0c63-4171-a249-945cae255c97/83eecfd4d79c74ee06c7d25d41334258
http://www.evernote.com/shard/s146/sh/e0d95bd1-68df-49d9-87c8-e21647d94e18/4d7af393bd986fd0c462ebd138f494b1