[多變] 偏微求極值一問
偏微分求極值時，判別式大於零之外，
還要看fxx如果>0就是極小，反之
請問為啥要看fxx不看fyy?
遇到一題變數不是x.y不知道該看哪個(不過都正)
e.g.
某兩點以參數式表示的距離d=√{(1+3t+2s)^2+(4-2t-3s)^2+(-4+t-s)^2}
求最短距離
偏微分解得(s,t)=(-1/5,4/5)
fss=28 ftt=28 fst=22
所以判別式>0 且 fss>0
所以此點參數會產生最小距離
我要問的就是怎麼是看fss而不是看ftt
--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 140.117.198.78
推
12/16 17:33, , 1F
12/16 17:33, 1F
→
12/16 17:34, , 2F
12/16 17:34, 2F
→
12/16 17:34, , 3F
12/16 17:34, 3F
→
12/16 17:34, , 4F
12/16 17:34, 4F
→
12/16 17:34, , 5F
12/16 17:34, 5F
→
12/16 17:35, , 6F
12/16 17:35, 6F
→
12/16 17:35, , 7F
12/16 17:35, 7F
→
12/16 17:51, , 8F
12/16 17:51, 8F
→
12/16 17:51, , 9F
12/16 17:51, 9F
→
12/16 18:01, , 10F
12/16 18:01, 10F
→
12/16 18:01, , 11F
12/16 18:01, 11F
※ 編輯: midarmyman 來自: 140.117.198.78 (12/16 18:36)
推
12/16 18:45, , 12F
12/16 18:45, 12F
→
12/16 18:45, , 13F
12/16 18:45, 13F
→
12/16 19:00, , 14F
12/16 19:00, 14F
推
12/16 19:04, , 15F
12/16 19:04, 15F
→
12/16 19:05, , 16F
12/16 19:05, 16F
→
12/16 19:05, , 17F
12/16 19:05, 17F
→
12/16 19:06, , 18F
12/16 19:06, 18F
→
12/16 19:06, , 19F
12/16 19:06, 19F
→
12/16 19:07, , 20F
12/16 19:07, 20F
→
12/16 19:07, , 21F
12/16 19:07, 21F
→
12/19 08:18, , 22F
12/19 08:18, 22F