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