P/s i need help to prove that every positive real no has a unigue positive square root
Any proof of this depends upon the set of definitions and axioms with the sequence of theorems you have.
Here is a general approach. If $\displaystyle c\in\mathbb{R}^+$ prove that if $\displaystyle 0<t~\&~t^2=c$ then there is at most one such $\displaystyle t$.
Let $\displaystyle E=\{x\in\mathbb{R}^+:x^2<c\}$. You want to show that $\displaystyle E\ne\emptyset$ and $\displaystyle E$ is bounded above.
If you can show those then use the completeness property.
Hint: let $\displaystyle t=\frac{c}{1+c}$. Is it true that $\displaystyle t\in E~?$
What can you do with that?
Plato, can you explain why this doesn't work? Given distinct real numbers x and y in R, suppose they have the same square root z. By definition, z = sqrt(x) and z=sqrt(y), so we have z^2=x=y which is a contradiction, so two distinct real numbers can not have the same square root.