du = ux`dx + uy`dy
ux` = 2x/(x^2-y^2-2^2) [此为u对x的偏导数]
uy` = -2y/(x^2-y^2-2^2) [此为u对y的偏导数]
所以du = 2x/(x^2-y^2-2^2) * dx - 2y/(x^2-y^2-2^2) * dy