calculate

where

M is a part of hyperboloid

which is located at 0<=x<=1 and 0<=y<=1 ,and its normal vector points outside

like this :

http://i28.tinypic.com/f9p63r.gif

i am used to solve it like this

now i convert into polar coordinates

x^2+y^2=r

how to what are the intervals for r

i just guessed its from 0 to 1

i dont know how to know the upper interval here

except that

is this method ok?