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你可以這樣想像一個z=f(x,y)的三維圖像,每一個(x,y)點都有一個z與之映射,可以想像得到那將是一個曲面,然後你想像曲面上一個特定的點,它就像你在爬山的時候站在半山腰一樣.如果你平的在那個半山腰左右走,那麼你的高…
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