y=ln(x-√(1+x^2)) 求dy/dx

设u=x-√(1+x^2)
dy/du=1/u
du/dx=1-x/√(1+x^2)
dy/dx=[1-x/√(1+x^2)]/[x-√(1+x^2)] =-1/√(1+x^2)