Why doesn't it work here ?
Whatever x is, the latter inequality can be restricted to :
, since .
So we have this new inequality :
which gives (*)
that is to say
note : , where b is positive.
(*) this can be proved.
so either and ----> x < a and x > -a. This is equivalent to |x|< a
either and -----> x > a and x < -a. This is not possible since a > 0.