y = Inu，u = x+√v，v = (x²+a²)，求导，得 dy/dx = (dy/du)*(du/dx) = (dy/du)*{1+[(1/2)/√v](dv/dx)} = {1/[x+√(x²+a²)]}*[1+x/√(x²+a²)] = 1/√(x²+a²)。
U > 0 考虑没有？
