为什么偏导数是法向量

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SELECT ArticleID,Title FROM tbl_Articles WHERE ArticleID IN(1083167,1072713,1056858,1064601,1073822,1084563,1082357,1051847,1071265,1078599,988780,1084391,1071701,1079950,1063165,1072679,1062618,1055866,1076241,1042765,1078438,1090150,1071501,1073222,1088431,1062424,1045655,1043355,1075648,1063917) ORDER BY field (ArticleID,1083167,1072713,1056858,1064601,1073822,1084563,1082357,1051847,1071265,1078599,988780,1084391,1071701,1079950,1063165,1072679,1062618,1055866,1076241,1042765,1078438,1090150,1071501,1073222,1088431,1062424,1045655,1043355,1075648,1063917)