I4.3 Let $c$ be a positive real number. If $x^{2}+2 \sqrt{c} x+b=0$ has one real root only, find the value of $c$
$\begin{array}{l}x^{2}+2 \sqrt{c} x+2=0 \\ \Delta=4(c-2)=0 \\ \Rightarrow c=2\end{array}$