What is the limit x-->0 (sqrt(1+x)-sqrt(1-x))/x ?

You should
substitute 0 for x such that:

`lim_(x->0) (sqrt(1+x) - sqrt(1-x))/x =
(sqrt(1+0) - sqrt(1-0))/0`

`lim_(x->0) (sqrt(1+x) - sqrt(1-x))/x = (1-1)/0
= 0/0`

The indetermination 0/0 could be solved using l'Hospital's
theorem...