21.3: 1, 3, 4, 5, 9

21.3: 5, 13 , 21, 23

21.3: 29 (prove by cutting the domain into 3 pieces without holes), 30, 31

Also: Explain why in problem 31 (and for any vector field with Nx=My) the value of the integral

on a closed curve does not depend on which point you consider to be initial/final point on the curve.