因为g'(y)=1/f'(x)=1/f'(g(y))
故根据复合函数求导得(注意y是自变量)
g''(y)=-f''(g(y))/f'²(g(y))*g'(y)=-f''(g(y))/[f'(g(y))]^3=-f''(x)/[f'(x)]^3