Write mathematical formula for:
Not every square root of a positive number is positive number (using $*,+,=,1,>,0$)
That is my solution that I am not sure about as I am not really good in these things:
$$\lnot \forall x(\exists k(x*x=k \land k>0) \Rightarrow x>0)$$
What about it?
I would write it in the following way, but yours is correct :)
$$\neg (\forall x\vert x>0\Rightarrow (\forall k\vert k*k=x \Rightarrow k >0))$$