\(A=\sqrt{\left(1-2\sqrt{5}\right)^2}-\sqrt{20}\)
\(=\left|1-2\sqrt{5}\right|-2\sqrt{5}\)
\(=2\sqrt{5}-1-2\sqrt{5}\)
\(=-1\)
\(A=\sqrt{\left(1-2\sqrt{5}\right)^2}-\sqrt{20}\\ =\left|1-2\sqrt{5}\right|-2\sqrt{5}\left(1\right)\)
\(Vì : 1<2\sqrt 5=>1-2\sqrt 5<0=>|1-2\sqrt5|=-(1-2\sqrt5)=2\sqrt5 -1\)
\(\left(1\right)=>2\sqrt{5}-1-2\sqrt{5}=-1\)
\(Vậy A=-1\)
\(=2\sqrt{5}-1-2\sqrt{5}=-1\)