f'(x) =2x/√(1-x^2)
d/dx (f(√(1-x^2))
= -(x/√(1-x^2)) f'(√(1-x^2))
=-(x/√(1-x^2)) [2√(1-x^2)/x]
=-2有点没看懂，d/dx (f(√(1-x^2))----> -(x/√(1-x^2)) f'(√(1-x^2)) 这一步d/dx(f(√(1-x^2))= f'(√(1-x^2).d/dx (√(1-x^2))=-(x/√(1-x^2)) f'(√(1-x^2))