x→0
lim (1-√(1-x))/x
=lim (1-√(1-x))(1+√(1-x)) / x(1+√(1-x))
=lim x / x(1+√(1-x))
=lim 1 / (1+√(1-x))
=1/(1+1)
=1/2
有不懂欢迎追问