Suppose C1 and C2 are physically the same curve, but they are parameterized so that the starting point of C1 is the ending point of C2, and
the ending point of C1 is the starting point of C2.
Express
∫ C1m(x, y)D „ x + n(x, y)D „ y
in terms of
∫ C2m(x, y)D „ x + n(x, y)D „ y.
∫ C1m(x, y)D ‚x + n(x, y)D ‚y = -∫ C2 m(x, y)D ‚x + n(x, y)D ‚y.