\(C=\sqrt{\left(8+2\sqrt{10+2\sqrt{5}}\right).\left(8-2\sqrt{10+2\sqrt{5}}\right)}=\sqrt{\left(8^2-\left(2\sqrt{10+2\sqrt{5}}\right)^2\right)=\sqrt{64-4\left(10+2\sqrt{5}\right)}}\)
\(C=\sqrt{64-40-8\sqrt{5}}=\sqrt{24-8\sqrt{5}}\)
\(C=\sqrt{20-2.2.2\sqrt{5}+4}=\sqrt{\left(2\sqrt{5}-2\right)^2}\)
\(C=2\sqrt{5}-2=2\left(\sqrt{5}-1\right)\)