then
where is the positive unit normal. If we write , then
Hence, for the choice of positive oriented normal on , given by and ,
Similar formula holds if is represented as or
Let the continuous normal have direction cosines i.e.,
Then, for
.
is the positive unit normal. If we write 
, then 
, given by 
and 
, 
is represented as 
or 
have direction cosines 
i.e., 
.
