\(A^2=4+\sqrt{10+2\sqrt{5}}+2.\sqrt{\left(4+\sqrt{10+2\sqrt{5}}\right).\left(4-\sqrt{10+2\sqrt{5}}\right)}+4-\sqrt{10+2\sqrt{5}}\)
\(A^2=8+2.\sqrt{4^2-\left(10+2\sqrt{5}\right)}=8+2.\sqrt{6-2\sqrt{5}}\)
\(A^2=8+2.\sqrt{5-2\sqrt{5}.1+1}=8+2.\sqrt{\left(\sqrt{5}-1\right)^2}=8+2.\left(\sqrt{5}-1\right)\)
\(A^2=6+2\sqrt{5}=5+2\sqrt{5}+1=\left(\sqrt{5}+1\right)^2\)
=> \(A=\sqrt{5}+1\) (Do A > 0)