最近在看regular surface的時後，遇到一系列要求檢驗是否為regular surface的問題，
我都卡在不知道如何證明面上所有的點都滿足regular的條件。
例如以下題目：
->
證明 r = (f(u)cos(v), f(u)sin(v), g(u))是regular surface，其中f(u)>0
我有算出來|r_u X r_v| = f√((f')^2 + (g')^2)
可是不知道怎麼證明根號內的(f')^2 + (g')^2在面上所有點均不為0
可以拜託板上強者教我一下如何證明它是一個regular surface嗎？
感謝回答~~
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