\(A=\sqrt{4-\sqrt{10-2\sqrt{5}}}-\sqrt{4+\sqrt{10-2\sqrt{5}}}\)
\(A^2=4-\sqrt{10-2\sqrt{5}}+4+\sqrt{10-2\sqrt{5}}-2\sqrt{16-10+2\sqrt{5}}\)
\(A^2=8-2\sqrt{5+2\sqrt{5}+1}\)
\(A^2=8-2\left(\sqrt{5}+1\right)\)
\(A^2=5-2\sqrt{5}+1\)
\(A=\sqrt{5}-1\)