如何用倒数的定义求函数y=x^（- 1/2）的导数?就是用{f(x+d)-f(x)}/d的方法求

d->0,
y'=lim([(x+d)^(-1/2)-(x^(-1/2))]/d)=lim{(x^0.5-(x+d)^0.5)/(x*(x+d))^0.5/d}
=lim{ (x-(x+d)) / [(x^0.5+(x+d)^0.5)*(x*(x+d)^0.5)*d]}
=lim{-1/[(x^0.5+(x+d)^0.5)*(x*(x+d)^0.5)]}
=-1/[x^0.5+x^0.5]/x
=-1/x^(3/2)/2
=(-1/2)*x^(-3/2)